OK, the replies above suggest what I'd thought. For any province (or given set of provinces) there is a given set of units that may or may not become available. Once hired, there is then a probability that one or other of the set units may become available (and there may be more than one type available at any one time)

Not exactly scientific, but at least it reflects my own experience.

So, to the modder types - is this info held in one of the text files somewhere? I seem to remember from MTW that merc probability was something that could be modded (or have I made that up?)

Cheers,

Rob.