Ruby has two separate Pokédexes to complete: the Hoenn Pokédex and the National Pokédex. All Pokémon from the Hoenn Pokédex, except for Jirachi and Deoxys (which were only obtainable via long-defunct events), 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 and Sapphire, all 200 Hoenn Pokédex Pokémon can be obtained. With access to Emerald, an additional 42 National Pokédex Pokémon (for a maximum of 242) can be obtained.
In order to complete the Hoenn Pokédex in Ruby, you will need access to at minimum a copy of Pokémon Sapphire. 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 Sapphire 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.
Pokémon that evolve via Moon Stone are listed here because only one Moon Stone can be obtained in Ruby. Access to Sapphire, where you can get infinite Moon Stones from wild Lunatone, will allow for more to be obtained.
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.
Six 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.