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>Red and Green are released in 1996
>Can complete the Pokédex 150/151 by trading between the two games, Mew isn't counted towards completion or found through normal gameplay, it's distributed to players later
>Gold and Silver are released in 1999
>Can reach 232/251 through gameplay, and 249/251 if you use the Time Capsule with previous games
>Mew and Celebi aren't available through normal gameplay but aren't required for completion
>Ruby and Sapphire are released in 2002
>Can reach 200/386 through gameplay, 6 Mythicals (Mew, Celebi, Lugia*, Ho-Oh*, Jirachi, Deoxys) not required for completion of the National Pokédex, 180 normal Pokémon completely unavailable for a year
>Diamond and Pearl are released in 2006
>Somewhere in the 440/493 range with dual slot mode, 11 Mythicals not required for completion of the National Pokédex, and with Pal Park, the games could reach 482/493 even if they don't have Mythicals
I can't remember Black and White, but it had backwards compatibility which means you could get 636/649 (not counting Mythicals) and it would count as a complete Pokédex
The 3DS games onward with Poké Transporter and Bank being required meant that the games temporarily had no backwards compatibility until those apps were released/updated, and it's been like that with HOME ever since. Not to mention with every Switch game missing large numbers of Pokémon by design.
