nOOb question # 11: Keeping track of killed units

[Keneth]

I don't think it says anywhere explicitly (at least I haven't noticed) but it's just simple math, especially if you have a full army. If you hover over the leadership requirement of the troop it tells you how many of them you can control - the number of current units. That's only if resurrection isn't available like if you've used the spellbook already or don't have it yet.