I know the number of men Stannis currently has is at around 5,000 but how many are Southrons, Mountain Clansmen, Mormonts, other Northern stragglers (Hornwood, Cerwyn,etc.)? Also, how many men do you estimate Stannis lost during the Battle of Castle Black?

Well, according to the wiki, it breaks down to:

– 1,200 Southrons.

– 2,500 hill clans.

– 1,200 Mormonts, Glovers, Hornwoods, Cerwyns, etc. 

– 400 Umbers.

Given the total surprise and the rout of Mance’s forces, being mounted vs. a largely infantry force, and superior discipline allowing them to maintain momentum and cohesion, I’d be surprised if Stannis’ army lost more than 50 men at the Battle for Castle Black.