Hypothetically? Not that many. The only ones really missing from X/Y are a handful of commons and a bunch of starters/legends.
The starters and commons can probably all be found between Black/White (I think I remember all the starters being available in those games, though to get them all you'd need to restart a lot.) All the legendaries are doubtlessly going to be handed out as events throughout the generation, so if you're patient you shouldn't need to bother with them.
Actually you can probably get the majority starting in gen 4. I think platinum has like 400+ and then you can get a lot of the legends from Soulsilver/Heartgold. And obviously you can get the unova dex in black and white. There may be a few exclusive to those generations but I'm sure you could trade for them.
Like the first Ranger for Manaphy. Luckily, I have the Colosseum Bonus Disc (Jirachi version, not Celebi version)
Even with those, there is still Deoxys,Darkrai,Shaymin,Arceus, and all the gen 5 events that don't rely on separate games to obtain them. It's still legitimately impossible to get all of them without attending events.
Freiza may be cool, but his brother is..... COOLER(get it?) 3DS FC:0645-6947-9076