If you ask for a quote to rekey locks, you will be asked how many cylinders you have.

A cylinder in a lock is the mechanism in which a key is inserted. If you have a lock that uses a key for the outside of the door, and has a thumb turn for the inside of the door, that is considered 1 cylinder.

If you need a key to unlock the door from the inside as well as from the outside, that is considered 2 cylinders.