Total mounts are 948 in game, Total obtainable mounts currently in game is 873. This number also varies based off the criteria for usable by single player. Realistically going up to 800 Mount achievement shouldnt be outside the realm of possibility for people to hit especially with the amount of mounts that have been added in the last 2 patches alone.
This would give people something to farm for for example using the Ur’zul model for say achievement 600 or something. This model for a mount currently only has one way to obtain. Gives players a unique reward and a reason to farm. Because right now the grind for recolors of the same mount is exhausting.