Where the 10 refers to the number of healthy men that are in your unit and the (+2) refers to the number of wounded. If the region you are in gets scouted, your unit will appear like this:
Footsoldiers 12 men
Wounded men do not count towards the number of men you can attack/defend with! They will sit the battle out, so remember to take that into consideration when playing BattleMaster.
Effect Wounded Men Have
The effect wounded men have is simple- they're wounded and thus cannot walk, run or ride as fast as they would if they where healthy. If you have more wounded men then healthy in your unit, your travel time will go up exponentially.
There are two main ways to handle wounded men in your unit.
The easiest is to wait. They will eventually get better or they will die. If you have a Healer with you, he can increase the odds of your men getting better, but it isn't a sure thing. (Plus he can get overwhelmed if there are more wounded men than he can treat)
Another way to deal with wounded men in your unit is to move to a region that has a "House of Healing" built in it. This will give you the option of leaving your wounded men in the hands of the Healers, some of them might return but the majority will stay.