The stats never reach 99.
You can gain a max of <number here> which adds 63 to your stat total.
Everything has a min-max according to the base stat and then you add IV's (0 min, 15 max), and stat exp. You gain stat exp according to the base stats of the opponent. So say you battle a Butterfree, you gain 80 special stat exp. (Butterfree's base special is 80.)
It works like that.
The highest possible stat is 703 HP by Chansey iirc.