Thank you Neavil for your response. I was having the same issue. Eleven toons, some have exclusive (which I know how that works) and I went to the links to check my mount counts. I even sat and made spreadsheets for each toon. (sad, I know.) Put in tickets, got information back that I already knew (some mounts are exclusive) but I was counting mounts usable on all characters. One explanation is that sometimes it awards the mount count instantly, sometimes it takes days, sometimes it does not award the count at all. It was also suggested that I change my password to my account to force a re-count of my character stats. Then I bought a mount (I was on my alt with the most mounts), and my count jumped up to the correct number. As mentioned in other comments, we shouldn’t be forced to have to spend our gold to get the ticker to count correctly. Neavil’s response got me thinking… so the next time I got a mount on another alt, I did not click on it. I mailed it to my highest mount character. Opened it as that toon, (the alt with the highest count, exclusive and normal mounts total) and the count corrected itself for all my alts. So I am going to do that from now on… if I earn/buy/questline a mount, I will mail it to the alt with the highest count and NOT open it as the alt that earned it. It’s messed up that we have to work so hard to get Blizz to give us credit for achievements they put out there for us to achieve, but I am hoping this work-around continues to work for me.
Still happening. My blood elf paladin is currently at 351 mounts (according to statistics on that character) It’s a little frustrating isn’t it. The whole having to add a specific mount to that one character that has the most mounts is pretty dumb, I don’t understand why it’s like that. If you add a mount on a alt it should be counted towards the achievement because it’s counted in the statistics…
That is a different situation. If you already had Sha’tari Defense at Exalted (on an Alliance character), getting Exalted with Laughing Skull Orcs (on a Horde character) will not count for a second reputation. They share credit as one reputation, just like other mutually exclusive reputations (Aldor/Scryer and Oracles/Frenzyheart).
Still an issue, but now they’ve removed the count from the stats page too, so you can’t double check that way anymore. Got my 200th mount, and checked the achieve and it was still at 196/200… brilliant work 
Having the issue too. I’ve gotten new mounts since shadowlands started, and the count has gone up. I’ve been at 299/300. went for ony and she dropped. she counts in the drake achievement tracker, but isnt my 300th mount. I’ve logged into every character I have, exited the game, and cleared my cache, and still, 299/300. what gives?
I got my 400th mount a wile ago. Today I got the Shimmermist Runner on my mage so I decided to log onto my paladin which I got my 400th mount on and her count dropped from 400+ down to 382. So I guess I will have to get another mount on the Paladin if I want the number to be corrected. I really hope they never add another achievement for mount collections until they fix this mess.
I went to the Wowhead tracker to check out how many unique mounts my pally had and that site not longer lists that count. I just shows a flat count of all mounts I have which is 452 of 694. In the past when you moused over the green bar that shows the flat count it would changed to show you the number of unique mounts that character could use. Now when you mouse over it just shows the percentage of the mounts you have, which in my case is 65%.Makes me wonder if they changed something in SL to mess things up even further with this.
Adding another “I’m having this issue, please help Blizz”.
Was at 398 a few months back and it went down to 394 when an update came out. I’ve since gotten at least 4 new mounts and the count never changes. I’ve logged into all my characters, disabled all add-ons, etc, etc etc - all the same things everyone else is already saying.
I submitted an in-game ticket and got a generic copy/paste response.
This is broken and it would be nice if we could at least get an acknowledgement of the issue instead of this garbage.
Greetings,
Thank you for contacting us in regard to an issue with an achievement. We are experiencing a high volume of tickets right now and apologize for being unable to discuss the issue further with you.
For achievement related issues, please attempt the following for potential resolution:
- Reset UI and retest with no add-ons: https://battle.net/support/article/7549
- Check the Wowhead database and guides for more information: https://www.wowhead.com/
- Visit the forums for questions and discussion: https:/us.forums.blizzard.com/en/wow/ or https:/eu.forums.blizzard.com/en/wow/ depending on the region of your choice
Game Masters are not able to grant achievement credit, provide game hints, or assist with in-game bugs. If you believe the issue is due to a bug, please follow the in-game steps noted below to notify our development team directly. In addition, visit our Bug Report forums to report bugs, discuss them with the community, and check the status of known issues: https:/us.forums.blizzard.com/en/wow/c/support/bug-report for current WoW or https:/us.forums.blizzard.com/en/wow/c/wow-classic/support-and-bug-report for WoW Classic
- Click the ? on your hotkey bar or press Esc on your keyboard
- Click Support to open the Customer Support window
- Click Submit feedback or bug report
- Enter the details of the bug into the Submit a bug Description box, making sure to follow the guidelines outlined on the submission form
- Click Submit
Your ticket was identified to be about the issue addressed above. If this is not the case, please respond so a Game Master can manually review it
I have the same problem. One way that I did find to get the count to bump up to the correct number was to buy a mount. That should not be the way to get the mount count to correct. I actually went to the Mount page and went through with a spread sheet to actually count my mounts manually by entering them into the sheet. Was exhaustive and time consuming because you have to scroll through pictures of each mount, and they included mounts from the other faction which I cannot use anyway. It was pointless because I was also given the generic response after all that work. So… I am going to have to spend gold to buy yet another mount just to get the count to correct itself. 
I’m having the opposite issue, sort of. I’ve restarted on a new account, only 1 character. My mount count is 20, but I only actually have 19 mounts. It’s been consistently off by 1 since the day I created my character. Bit frustrating as I’d like an accurate count.
I have the same issue, I’m stuck in 354 mounts, the count is not working since last week, and i already got 3 new mounts, including the horse that comes in Venary’s box, the Ellusive Quickhoof and MechaMogul MK2 i bought from AH today…please Blizz fix it.
Today my counter for mount achievement was 355. Then i went to Arathy Highlands and got 2 new mounts: Broken Highland Mustang and Lil’ Donkey. But the counter become 356…thats ridiculous, should be 357…
This glitch is still happening as of Jan 22 2022. I have 275 mounts on a single character that I can use, yet the achievement for me says 236. I just got two more mounts and the achievement still stays at 236. Fix this please.
This is still an issue. I was at 245 mounts, bought the wen lo mount and opened it on my Paladin and it stayed at 245. I then logged into every character across my account and they were all at 245. I just so happened to get the honeyback harvester mount a couple days later and the total went to 246.