What would the tooltip give more than the amount of troops ? Which is also visible next to the unit.
Composition of a stack, in fact just symbols above the flag would be even better so you don't have to hover the mouse. Horse-head silhouettes for cavalry, "HQ" for... guess. Rifles for infantry, a spade for engineers (if they are in the game of course) that kind of thing.
Gold symbols for vets, silver for green, bronze for drafted units...