Just hower your mouse over the stack and check if any brigade has "room" (as in my screenshot above). At least I tend to do that regularily
The problem there is that replacements are not the only reason for that set of numbers to vary. At least, I'm pretty sure that it's not. The times that I've noticed the numbers changing is when the units cohesion or strength isn't at 100%.
Also, you'd have to separate stacks to receive accurate info on specific units.
Point being it would simply be nice for things to be clearer. None of this is game breaking by any means, but it would make the game easier to use and manage. Since it sounds like Pocus agrees, hopefully we'll get something in a patch... eventually.