There are 718 Pokemon in total. According to Serebii.net, there are roughly 125 (I may have miscounted) that can't be obtained in either X or Y. So with a mixture of catching, trading and breeding, you should be able to obtain about 593 species.
The rest will eventually become available in December when the Pokemon Bank launches. I haven't read into it yet, but from what I've heard so far it sounds like a cloud storage app that lets you trade between Gen IV, V and VI games.
Sarcasm is sooo hard to convey in a written format.
Before the Elite Four, there are 450 Pokemon native to Kalos. After it, a few Legendaries become available, and a new city opens up that has a Friend Safari, which allows you to capture more Pokemon that are not Kalos native. There's probably 550 or so total catchable pokey-men in X and Y alone, out of the total of 718.
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