Oddly it seems that question hasn't been asked yet. The Warpedia says:
Units with ambush value have a chance to ambush the enemy and kill him before entering a fight. When the ambush hit or missed, that unit won't get another ambush chance until the next time you attack with that unit
I'm not sure of the meaning of "attack" in this context. Does it mean that any given unit can use ambush ability only once during a battle? For instance, let's say my orc kills an opponent unit with ambush in the first battle round. Does it use ambush on the next unit in the following round or only in the next battle?
If it's the latter, then it sucks to be orc