Originally Posted by
Azerate
For whatever reason, they decided to remove that information from the statistics.
If you want the "mounts usable on a single character" count you will need to get that through addons. I downloaded one of them to test it because of this thread and sadly the counter it shows seems broken as it tells me I have 515 and I had more than 540 even before SL launch. I'm guessing it's not counting legion class hall mounts and probably something else as well. So until the achievement actually gets released you're out of luck.
It also seems the main counter that shows up without addons now includes mounts that are not usable on a single character, but doesn't include other faction mounts maybe? Since it's also lower than my total count for all mounts cross faction.
In other words, it's a big mess and Blizzard removed the one and only reliable counter that allowed collectors to track the number. It's like them saying "if you have the highest achievement, you shouldn't care".