Normally you should...
every enemy which is only "wounded" during the battle,
should be available as prisoner after the battle,
unless, as Gaunt already said, there were allies in the battle
(the exception being lords of course, as they have a chance to escape)
You can also order your soldiers to only use blunt weapons
(in the same menu that has "Fire at will" and "Cease firing")...
this will ensure that you get many prisoners during the fight,
although it might also result in more losses for your army.