Since we're getting 400 mons in the dex, making it just 50 shy of Kalos, I think this game has the potential to have the largest amount of mons available in one pair of games, atleast in the 3ds era, with the reasoning being that they'll most likely add in the starters and legendaries like ORAS,
If they couple that with any single new method of finding pokemon in the postgame (Like Friend Safari in XY, dexnav in ORAS,Swarms, Radar, whatever new areas are hiding behind the clouds, etc.) that should be enough to break the biggest record we have so far

GF said in an interview recently they'd like to keep the full national dex completed in every generation so that new players arent relient on old games and bank to catch em all, I dont quite think we'll get the FULL dex in the game (It could potentially be possible since its the last 3ds games and they wanna hold us over until the switch games) but I think the full total will be around 500-600

XY probably would have taken the record if it had a third version or sequel which expanded its availability, but since the new Alola Dex is just about as big, it's probably gonna take the record