Six prisoners for a city the size of King’s Landing seems odd to me. Do you think imprisonment is being used as a punishment for any crimes, or are they more likely simply being held pending someone deciding what parts to chop off of them?

Given that the Westerosi tend to go in for physical punishments – steal and you lose a hand, rape and you get gelded, kill someone and be hanged, etc. – they probably don’t have a very large penal system. 

Also, it’s six prisoners who Yoren thinks would make acceptable material for the Watch, so he’s going to reject anyone who’s too disabled, too old, etc.