Sapphire has two separate Pokédexes to complete: the Hoenn Pokédex and the National Pokédex. The Hoenn Pokédex contains 200 Pokémon (202 counting Jirachi and Deoxys), which includes every Pokémon introduced in Ruby, Sapphire, and Emerald, while the National Pokédex consists of the Hoenn Pokédex plus the other Pokémon introduced in Red, Green, Blue, & Yellow and Gold, Silver, and Crystal (Pokémon 1-251). All Pokémon from the Hoenn Pokédex, except for the event-exclusive Jirachi and Deoxys, are available between Ruby, Sapphire, and Emerald. Only a select few Pokémon from the National Pokédex are available, and only in Emerald, making it impossible to complete the National Pokédex with access to only those three games. This page will show the maximum number of Pokémon that can be obtained.
Between Ruby, Sapphire, and Emerald, a total of 242 Pokémon can be obtained (200 Hoenn Pokédex Pokémon plus 42 National Pokédex Pokémon in Emerald). If the player has access to Navel Rock, Birth Island, and Faraway Island in Emerald, they would be able to get an additional 4 Pokémon.
In order to complete the Hoenn Pokédex in Sapphire this way, you will need access to at minimum a copy of Pokémon Ruby. In order to get the Pokémon added to the National Pokédex in Emerald, you will obviously need access to Emerald as well.
The following version-exclusive Pokémon have to be traded from Ruby or Emerald:
The following National Pokédex Pokémon have to be traded from Emerald:
Several Pokémon are mutually exclusive, meaning that if you get one species, you cannot get at least one other species. The list of mutually exclusive species is below.
You will need access to three save files in order to get all three starters. You will also need access to three Emerald save files in particular to collect all three Johto starters, and each Emerald file will need to be able to complete its Hoenn Pokédex.
Only one Pokémon that evolves by Sun Stone can be obtained in Ruby: Bellossom. However, Sunkern, which is accessible in Emerald, also evolves by Sun Stone. If you are trading a Sunkern from Emerald, be sure to evolve it into Sunflora first, since Emerald can get more than one Sun Stone from wild Solrock.
Ten Pokémon evolve by trading: Alakazam, Golem, Machamp, Huntail, Gorebyss, and Kingdra. The easiest way to get data for them is to trade them to another game and immediately trade
the newly-evolved Pokémon back.
You will need one Deepseatooth, one Deepseascale, and one King's Rock to evolve Clamperl and Seadra. The player can only choose one of the Deepseatooth and Deepseascale per save file, meaning that only one of Huntail or Gorebyss can be obtained on a given file.