The Kalos Pokedex was divided into three separate sections each of which contained 150 entries, plus legendaries enumerated beyond 150. That doesn't mean a single game can get all 450, of course, because of version exclusives, trade evolutions, and branched choices (the only notable ones being Tyrunt vs. Amaura, as all the other fossils you can get in those games are renewable pickups, and Articuno vs. Zapdos vs. Moltres which depends on the original starter you picked). There's also an XY-exclusive feature called the Friend Safari, which allows you to find some Pokemon, even those that would otherwise be version-exclusive, and a few that aren't in the Kalos Pokedex at all, based on the specific friend codes you have registered. This feature also enables you to catch all the starters from regions that you get an in-game starter choice for, so that ceases to be a limiting choice.
In all, assuming the ideal friend list setup, a single Pokemon X file without access to trading, events, or importing anything from elsewhere such as Bank has access to 557 different species, which beats out Y by a few simply because the Friend Safari does more to cover for Y-exclusives than it does for X-exclusives (you can get a Cloyster there but not a Starmie, for instance; a Pupitar but not a Lairon; a Manectric but not a Houndoom).
OR/AS are each capable of getting 407 different species, and the number is properly balanced on both sides there. This includes the Pokemon that are only available at the randomly appearing mirage islands, and the ones that only appear by popping up out of the grass or water at the end of the game where you have to sneak up on them.
Original Sun/Moon are each capable of getting 351 different species, which includes not just the Alola Pokedex but also Island Scans. A different scan is available per island per day, and they're all 3-stage evolution lines, so that would be worth 84 but Horsea and Rhyhorn are incapable of getting to their final stage without trading. Conkeldurr is a trade evolution too, but fortunately in that case you actually catch it directly in that form so you can still check it off the list (even if the game doesn't give you such a list to check it off of, with no National Dex and all).