Ok, I just worked out that with the current mounts plus the new flying carpets, my mage can have a total of 49 epic mounts, both land and flying. 50, if I was to grind the materials to get a Dark Riding Talbuk, or 51 if I am able to get the Brewfest mount next time it comes around. This also includes PvP mounts, which will be a total of 90 WSG and AB, and 140 AV tokens.
That leaves a minimum of 26 new mounts from the expansion. This shouldn't be too hard to get, considering there are new factions and will be new drops. And if I somehow managed to get a rare drop from somewhere, that would be one less mount to grind for or purchase. ;)
Of course, if by 75 mounts they also mean rare mounts, and rare mounts are not overwritten by epic mounts, then those 26 are already there for the taking. I would think that they mean all mounts and that you can have both rare and epic, even if you were to never use the rare ones.
I look forward to gaining Leading the Cavalry and my rare Albino Drake! IMO, it should increase flight speed by 310% and the mounts that count towards your 75 should have to be of epic quality -- provided there will be enough new mounts in the expansion; otherwise it will be impossible for anyone to get this achievement, considering how many more I -- a tailor with more mounts available to me -- must achieve.
Gaiwyn of Proudmoore