Ingame mount count - Community / General Discussion - World of Warcraft Forums (blizzard.com)
Some help there for an addon i linked.
Problem with buying all factions mounts on both sides is that the only count as one as a undead horse counts as a human horse so in realitiy one mount.
I am up to 608 mounts atm and there is until 9.2 another 120ish mounts i can grind/collect and then another 70 coming with 9.2.
Hitting 400 mounts now should be easyish
Daily/weekly dungeon & raid mounts guide - Guides - Wowhead
Lord of the Reins: The Complete Mount Collecting Guide - Guides - Wowhead
Mount Collecting: A Completionist Guide - Guides - Wowhead
Shadowlands Mount Guide - Guides - Wowhead
I hope they help, with collecting its about time/camping/gold/luck and bags of patience 