I know I took onager casualties and was down to 4 troops, usually I get 2 onagers per unit, but with 4 men I only had 1. What amount is required to bring a 2nd back in, I dont know as cant gradually increase troops, but if your on a front line with them and and get caught with archers b4 you can withdraw them, it helps to have some extra men in reservers, as non of my outbound territores could replace troops for siege engines, and they take forever to get back.