Statistics 4 should reveal how many armies are available in a mercenary camp before getting it.
Do you mean
A mercenary camp? Then this is one of the two numbers that describe each camp: In
₳189M available for ₩17 each the
₳189M is the maximum number of armies available in this camp.
If you mean
ALL camps in total, then sure, this would be a nice information but probably too primitive for statistics, this then should just be available for everyone, I guess.