So every character on my two accounts linked to my Bnet show either 358 or 359 mounts owned. The achievement however only shows 309 on each character.
Yesterday I startred the day off at 308 mounts. through the day I collected the following mounts:
Subdued Seahorse
Long Forgotten Hippogryph
Riddlers Mind-worm
3 + 308 = 311
Yet my achievement lists 309 across all characters on all accounts. So what gives?
Although it shows you own a number of mounts, only some characters can actually use those, so the count is based on that number. (Class, profession, faction, covenant etc)
However, there’s a bug that has existed for over a year that you have to learn the mounts on the character with the highest useable mounts for the count to go up properly. If you learn one on a character that can only use 308 but you’re count is at 309, it will stay at 309. Sometimes that particular character may be several mounts behind and it will take much longer to exceed the count that’s tracked from another character. It used to be that you had to just log into the character with the highest useable mounts but that no longer works.
At certain intervals you’ll have to continue to learn mounts on the one that can use the highest amount to bring that number back up, but sometimes figuring out which one that is can be tricky. A character with tailoring or engineering or more base class mounts (like your warlock or a Paladin) may be likely to have more.
So you are saying I have to log in to all 20+ characters and find the one with the highest owned mounts and then learn a new mount?
Yes, that seems to be the only way to trigger the updates. It can be tricky to figure out who has the most, so it may take some trial and error if you’re not sure. The mount count per character was removed from the statistics panel around when Shadowlands launched, which doesn’t help. I’ve yet to find a website that tracks this in a usable fashion. Maybe someone else will have some advice in that regard! It might be time for a new bug report about the issue, too, since it’s been awhile.
Yeah I found it, a toon I haven’t played in forever has one more mount than the others. A hunter that is lvl 50 and completely naked and broke. : /
Thanks for your help. 
Haha. Hooray! Glad to help! If I may ask, how did you quickly figure out each character’s total? It may be useful if anyone else finds this thread, but I’d also like to know if there’s a trick
I literally started logging them in, server by server till I found the one with the highest.
Got it, I am not in game at the moment, so I’m guessing the number may show in the mount panel somewhere? I will take a look later when I am.
I checked the Mounts panel yes. All over my toons showed 360 mounts except the hunter, she showed 361. But only 309 towards the collection as all my other toons show.
Ok thank you! I haven’t noticed that before! Good to know I don’t have to make a complicated spreadsheet lol
So now all I have to do is acquire a mount on that toon and it will update everything?
It should work if that is the one with the most mounts, from my experience and others I’ve seen on the forum. Definitely report back with your experience, either way.
Lol, a lvl 50 naked toon. And on a server I totally cannot stand being on. This should be fun. Thanks!
Haha. Good luck to you!
So I purchased a store mount and opened it with the toon with the most mounts. It updated the mount count on the achievement to reflect the new mount. It has me at 314 of 350 even though the toon now has 364 mounts. So something is not right here.
I’m glad you got your achievement count to update. I think the issue is that the count in-game in the mount tab includes mounts that are not usable by a character, such as opposite faction mounts, whereas the achievement ignores those.
Hrmmm, now this sounds like it might be the case actually.