Currently according to the Collections Bot on the WoW Secret Finding & Collections Discord Server, I have 568 usable mounts on my main and 621 account-wide. This, if you count my Alliance and Horde collections combined.
If you are looking for ways to farm mounts, You may want to ask in This Thread from the Achievements Forum, As there is many collectors there that may be able to guide you in the right direction.
Two helpful Discord Servers that will help you obtain mounts!:
Server Name: WoW Achievements
Server Link: https://discord.gg/8tQGtESC
Server Name: WoW Secret Finding & Collections
Server Link: https://discord.gg/wowsecrets