LeafGreen has two separate Pokédexes to complete: the Kanto Pokédex and the National Pokédex. All Pokémon from the Kanto Pokédex, except for Mew (which was only obtainable via long-defunct events), are available between FireRed and LeafGreen. Only a select few Pokémon from the National Pokédex are available, making it impossible to complete the National Pokédex with access to only those two games. This page will show the maximum number of Pokémon that can be obtained.
Between FireRed and LeafGreen, all 150 Kanto Pokédex Pokémon can be obtained, and a total of 212 National Pokédex Pokémon can be obtained. If you are playing on the Switch rerelease, you are able to get Ho-Oh, Lugia, and Deoxys, making the total instead 215.
The following version-exclusive Pokémon have to be traded from FireRed:
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.
Both of Hitmonlee and Hitmonchan, and all of Eevee's stone evolutions, can be obtained in both games without trading through the use of breeding. Espeon and Umbreon cannot be obtained as FireRed and LeafGreen have no day/night cycle.
Only one of Raikou, Entei, or Suicune can appear on any given save file. Which beast is available depends on the starter Pokémon picked: choosing Bulbasaur results in Entei, Charmander in Suicune, and Squirtle in Raikou. In order to get all three beasts, at least three save files are needed in FireRed and LeafGreen. Three save files are already needed to get all three starters, but to get the beasts, the player in each save file will need to progess as far as defeating the Elite Four and delivering the Sapphire to Celio on One Island.
Ten Pokémon evolve by trading: Golem, Machamp, Alakazam, Gengar, Politoed, Slowking, Steelix, Scizor, Kingdra, and Porygon2. 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 two King's Rocks, two Metal Coats, one Dragon Scale, and one Upgrade to evolve Politoed, Slowking, Steelix, Scizor, Kingdra, and Porygon2.