As others have explained the achievement is per character and how many mounts they can actually use. I tend to use my paladin as she has the most mounts available to her. The mounts tab in game shows a lot higher number, for some reason it started counting mounts you have unlocked for your faction even if you can’t actually use them.
This is a really handy site to see what an individual character has and is missing
You just put in your realm and character name.
For example this would be me -