Do Mercenaries replenish their ranks?

[Sencha]

If I hired a mercenary that had 30 units, and at the end of their employment, they were down to say, 15, do they get restore their numbers at some point?

I noticed that if I hire a mercenary group more than once, they will have the same amount of units that were left after the first hiring. So does that mean a mercenary group can be entirely defeated and removed from the game?