How Many Pokémon Can You Get with Only Pokémon Crystal?
Assuming you do not use any glitches, the answer is 206. The Professor Oak challenge guide for Crystal can be found here, although I've made a shorter list of encounters below in case anything happens to the external guide.
If you are playing the 3DS Virtual Console rerelease of Crystal, you will have access to Celebi with the GS Ball item. If you can get Celebi, the total is instead 207.
The list of Pokémon you cannot get without trading, which equals 52 species, is as follows:
- Two Starters (6) - 6 total
- As you can only select one starter Pokémon, the other two starters, and their evolutions, are unavailable; this totals 6 Pokémon.
- Trade Evolutions (10) - 16 total
- Alakazam, Machamp, Golem, Gengar, Politoed, Slowking, Steelix, Scizor, Kingdra, and Porygon2 evolve by trade. Without the ability to trade, they are unobtainable.
- Version Exclusives (10) - 26 total
- The following Pokémon cannot be obtained without the use of glitches or trading: Mareep, Flaaffy, Ampharos, Vulpix, Ninetales, Mankey, Primeape, Girafarig, Remoraid, and Octillery.
- Red, Green, Blue, or Yellow Pokémon (17) - 43 total
- Bulbasaur, Ivysaur, Venusaur, Charmander, Charmeleon, Charizard, Squirtle, Wartortle, Blastoise, Omanyte, Omastar, Kabuto, Kabutops, Articuno, Zapdos, Moltres, and Mewtwo can only be obtained from Red, Green, Blue, or Yellow.
- Mythical/Event (2) - 44 or 45 total
- Outside of glitches, Mew was only obtainable via events that are no longer active. The Virtual Console rerelease of Crystal provide access to Celebi, but original Game Boy Color cartridges cannot get Celebi outside of events that are no longer active.
Professor Oak Challenge Notes
Pre-Badge 1 (Falkner) - 46 Total Pokémon
- Gift in New Bark Town: One starter & its evolutions (3 total)
- Walking on Route 29: Pidgey, Pidgeotto, Pidgeot (6 total)
- Walking on Route 29: Sentret, Furret (8 total)
- Walking on Route 29: Hoothoot, Noctowl (10 total)
- Walking on Route 29: Rattata, Raticate (12 total)
- Walking on Route 29: Hoppip, Skiploom, Jumpluff (15 total)
- Walking on Route 46: Geodude, Graveler (17 total)
- Walking on Route 46: Phanpy, Donphan (19 total)
- Walking on Route 46: Spearow, Fearow (21 total)
- Walking on Route 30: Caterpie, Metapod, Butterfree (24 total)
- Walking on Route 30: Weedle, Kakuna, Beedrill (27 total)
- Walking on Route 30: Ledyba, Ledian (29 total)
- Walking on Route 30: Spinarak, Ariados (31 total)
- Walking on Route 30: Zubat, Golbat, Crobat (34 total)
- Walking on Route 30: Poliwag, Poliwhirl (36 total)
- Walking on Route 31: Bellsprout, Weepinbell (38 total)
- Walking on Route 31: Gastly, Haunter (40 total)
- Walking in Dark Cave (Route 31/46 side): Teddiursa, Ursaring (42 total)
- Walking in Dark Cave (Route 31/46 side): Dunsparce (43 total)
- In-game Trade in Violet City: Onix: Trade your Bellsprout (44 total)
- Walking on Route 36: Growlithe (45 total)
- Walking in Ruins of Alph Inside: Unown (46 total)
Badge 1 through Badge 2 (Bugsy) - 78 Total Pokémon
To use Headbutt here before being able to use Cut, you will need to use Slowpoke, who learns Headbutt at level 34.
If your character has a Trainer ID ending in 7, you will not be able to get Heracross until Route 42.
- Gift (Egg) in Violet City: Togepi, Togetic (48 total)
- Walking on Route 32: Wooper, Quagsire (50 total)
- Walking on Route 32: Ekans, Arbok (52 total)
- Fishing (Old Rod) in New Bark Town: Magikarp, Gyarados (55 total)
- Fishing (Old Rod) in New Bark Town: Tentacool, Tentacruel (57 total)
- Fishing (Old Rod) in Cherrygrove City: Krabby, Kingler (59 total)
- Fishing (Old Rod) in Dark Cave (Route 31/46 side): Goldeen, Seaking (61 total)
- Fishing (Old Rod) on Route 32: Qwilfish (62 total)
- Walking in Union Cave 1F: Sandshrew, Sandslash (64 total)
- Walking in Slowpoke Well 1F: Slowpoke, Slowbro (66 total)
- Walking in Ilex Forest: Oddish, Gloom (68 total)
- Walking in Ilex Forest: Paras, Parasect (70 total)
- Walking in Ilex Forest: Psyduck, Golduck (72 total)
- Walking in Ilex Forest: Venonat, Venomoth (74 total)
- Headbutt (Common, Rare) on Route 29: Exeggcute (75 total)
- Headbutt (Rare) on Route 29: Pineco (76 total)
- Headbutt (Common, Rare) in Azalea Town: Aipom (77 total)
- Headbutt (Rare) in Azalea Town: Heracross (78 total)
Badge 2 through Badge 3 (Whitney) - 111 Total Pokémon
All of the Pokémon that can be hatched from the Odd Egg are available by breeding. Choosing to hatch Tyrogue simply means you have access to more Pokémon before Badge 3 than you otherwise would, since you can get four Pokédex entries.
- Walking on Route 34: Drowzee, Hypno (80 total)
- Walking on Route 34: Abra, Kadabra (82 total)
- Walking on Route 34: Ditto (83 total)
- Walking on Route 34: Snubbull, Granbull (85 total)
- Walking on Route 34: Jigglypuff (86 total)
- Gift (Odd Egg) on Route 34: Tyrogue & one evolution (88 total)
- Route 34: Breed Jigglypuff to get Igglybuff (89 total)
- Route 34: Breed Tyrogue's evolution to get two Tyrogue, and evolve them into the other two evolutions (91 total)
- Purchase in Goldenrod Game Corner: Cubone, Marowak (93 total)
- Purchase in Goldenrod Game Corner: Wobbuffet (94 total)
- In-game Trade in Goldenrod Department Store 5F: Machop, Machoke: Trade your Abra (96 total)
- Walking on Route 35: Yanma (97 total)
- Walking in National Park: Nidoran♀, Nidorina (99 total)
- Walking in National Park: Nidoran♂, Nidorino (101 total)
- Walking in National Park: Sunkern (102 total)
- Walking in the Bug-Catching Contest: Scyther (103 total)
- Walking in the Bug-Catching Contest: Pinsir (104 total)
- Evolve Gloom and Sunkern with Sun Stones from the Bug-Catching Contest: Vileplume, Sunflora (106 total)
- Evolve a Moon Stone Pokémon with a Moon Stone from the player's mom (107 total)
- Evolve Exeggcute, Gloom, and Weepinbell with Leaf Stones from Picnicker Gina on Route 34 (110 total)
- Evolve Growlithe with a Fire Stone from Schoolboy Alan on Route 36 (111 total)
Badge 3 through Badge 4 (Morty) - 143 Total Pokémon
- Static encounter on Route 36: Sudowoodo (112 total)
- Walking on Route 37: Stantler (113 total)
- Gift in Goldenrod City: Eevee, Espeon (115 total)
- Route 34: Breed Espeon to get Eevee, and evolve it into Umbreon (116 total)
- Evolve a bred Eevee with a Fire Stone from Schoolboy Alan on Route 36 (117 total)
- Walking in the Burned Tower 1F: Koffing, Weezing (119 total)
- Walking on Route 38: Magnemite, Magneton (121 total)
- Walking on Route 38: Miltank (122 total)
- Walking on Route 38: Tauros (123 total)
- Walking on Route 38: Meowth, Persian (125 total)
- Evolve a bred Eevee with a Thunderstone from Lass Dana on Route 38 (126 total)
- In-game Trade in Olivine City: Voltorb, Electrode: Trade your Krabby (128 total)
- Rock Smash on Route 40: Shuckle (129 total)
- Fishing (Good Rod) in Olivine City: Corsola (130 total)
- Fishing (Good Rod) in Olivine City: Staryu (131 total)
- Fishing (Good Rod) in Olivine Harbor: Shellder (132 total)
- Fishing (Good Rod) in Olivine Harbor: Chinchou, Lanturn (134 total)
- Walking in Mt. Mortar: Marill, Azumarill (136 total)
- Evolve Shellder, Staryu, Poliwhirl, and a bred Eevee with Water Stones from Fisher Tully on Route 42 (140 total)
- Walking on Route 43: Farfetch'd (141 total)
- Roaming Legends: Raikou, Entei (143 total)
Badge 4 through Badge 5 (Pryce) - 150 Total Pokémon
- Evolve a Moon Stone Pokémon with a Moon Stone from Tohjo Falls (144 total)
- Walking in the Ruins of Alph Outside: Natu, Xatu (146 total)
- Walking in the Ruins of Alph Outside: Smeargle (147 total)
- Static encounter in Union Cave B2F: Lapras (148 total)
- Evolve a Moon Stone Pokémon with a Moon Stone from the Ruins of Alph (149 total)
- Surfing on Route 41: Mantine (150 total)
Badges 5 through Badge 7 (Chuck) - 154 Total Pokémon
Badge 7 through Badge 8 (Clair) - 170 Total Pokémon
- Static encounter in Bell Tower 1F: Suicune (155 total)
- Walking on Route 44: Lickitung (156 total)
- Walking on Route 44: Tangela (157 total)
- Walking in the Ice Path: Jynx (158 total)
- Walking in the Ice Path: Delibird (159 total)
- Walking in the Ice Path: Swinub, Piloswine (161 total)
- Walking in the Ice Path B1F, B2F, and B3F: Sneasel (162 total)
- Walking on Route 45: Gligar (163 total)
- Walking on Route 45: Skarmory (164 total)
- Fishing (Good Rod) on Route 45: Dratini, Dragonair, Dragonite (167 total)
- In-game Trade in Blackthorn City: Dodrio: Trade your female Dragonair (168 total)
- Route 34: Breed Jynx to get Smoochum (169 total)
- Route 34: Breed Dodrio to get Doduo (170 total)
Badge 8 through Elite Four - 174 Total Pokémon
- Walking on Route 27: Ponyta, Rapidash (172 total)
- Walking in Victory Road: Rhyhorn, Rhydon (174 total)
Elite Four through Badge 16 - 203 Total Pokémon
If you are able to obtain Celebi, the total for this section is 204.
- Static encounter in Ilex Forest: Celebi
- Walking on Route 10: Electabuzz (175 total)
- Walking in Rock Tunnel B1F: Kangaskhan (176 total)
- Walking on Route 7: Houndour, Houndoom (178 total)
- Walking on Route 7: Murkrow (179 total)
- Walking on Route 16: Grimer, Muk (181 total)
- Walking on Route 16: Slugma, Magcargo (183 total)
- Purchase in Celadon Game Corner: Porygon (184 total)
- Purchase in Celadon Game Corner: Larvitar, Pupitar, Tyranitar (187 total)
- Walking on Route 14: Chansey, Blissey (189 total)
- In-game Trade on Route 14: Aerodactyl: Trade your Chansey (190 total)
- Static encounter in Vermilion City: Snorlax (191 total)
- Walking in Diglett's Cave: Diglett, Dugtrio (193 total)
- Walking on Route 2: Pikachu (194 total)
- Walking in Mt. Moon: Clefairy (195 total)
- Evolve Clefairy with a Moon Stone from the Clefairy event in Mt. Moon (196 total)
- Walking on Route 21: Mr. Mime (197 total)
- Route 34: Breed Electabuzz to get Elekid (198 total)
- Route 34: Breed Pikachu to get Pichu (199 total)
- Route 34: Breed Clefairy or Clefable to get Cleffa (200 total)
- Static encounter in the Whirl Islands B3F: Lugia (201 total)
- Static encounter on Bell Tower 10F: Ho-Oh (202 total)
- Evolve Pikachu with a Thunderstone from Bill's grandfather on Route 25 (203 total)
Badge 16 through defeating Red - 206 Total Pokémon
If you are able to obtain Celebi, the total for this section is 207.
- Walking in Mt. Silver 1F: Magmar (204 total)
- Walking in Mt. Silver 2F and item rooms: Misdreavus (205 total)
- Route 34: Breed Magmar to get Magby (206 total)