Thing is, the number could be placed in multiple places. And it could be baked into other numbers too. And then there is the fact that it is highly likely to have been used for static memory allocation, something that tends to be a hassle to redo.
Well, I have three guesses on how it works:
1) checks what is the "actor limit" in X address and only adds actors up to that limit when called.
2) counts down instead. As in: the less enemies on screen, the higher the value. So, with 5 enemies (counting supply troop), the value would be at its lowest.
3) assigns a different value to each address, as to confirm whether it is a swordman, spearman, archer, supply troop, officer with X weapon and so on, or general. This is a possibility too.
All I know is: there must be a limiter somehow, such limiter not used in certain games, like Tyrian 2000 GB, Gradius and some others. That I know.