A little after Antietam the Cavalry Division could only mount 800 men to counter Stuart.
It's surprisingly common, Sherman killed virtually all his cavalry and artillery horses in the march to the sea...
Source? (on both, ideally)
That seems fairly low, unless you mean right after the epidemic that inconveniently broke out in time to give McClellan "my cavalry needs horses" to his list of reasons why his army couldn't pursue an army in even worse shape than the AotP could possibly be described as being - and that's giving all due credit for this not being anywhere near its best.
Though, I'd give a great deal while asking to know if that was "only 800 men had mounts" or "only 800 men with mounts were available" - because given McClellan's poor use of cavalry (seriously, regiments attached to divisions or corps in piecemeal fashion, with an army reserve of barely a large brigade?), it wouldn't surprise me if it was the latter even allowing for a severe horseflesh shortage.
As for Sherman...that I find harder to believe.
And this interesting:
http://ehistory.osu.edu/osu/sources/recordview.cfm?content=/027/0067
Note the strength of the Cavalry Division vs. the "strength in action" as the 4,320.
And the total army strength.
Since this is from McClellan, who is not likely to have overcounted his strength to any great extent, particularly when trying to argue that he's facing superior forces.
Very interesting.