[thejeff]

Army size, yes, but from the explanation in the manual that seems to apply to sneaking troops, not commanders.

I don't think it explicitly states each commander (and any squads he commands) are dealt with separately, but it is implied.

And I know I've seen only one of several scouts get caught.

I'd guess it's one patrol roll compared to a sneak roll for each commander. This produces reasonable results in normal conditions when the chances are decent, but when a really good open ended roll is required, you'll either catch none or all.