the unit can consist of 1 man, when he is part of a larger army. If you take this unit and separate it, it will automaticly gain 1 person, the captain.
dont know how they can grow to 4 though.

after a fight, you can see how many men have healed their wounds. For instance you would have 50 men when the fight ended, but when you check the stats after the fight you have 55 and 5 healed
perhaps some need longer to heal and rejoin the army? That would be pretty impressive and funny