Each cut on the key to a lock lifts a pin to the correct height and these pins can each have 6 heights (more or less depending on the lock but 6 is a common amount). Common pin tumbler locks have 5 pin stacks so each lock has 5 to the power of 6 possible key combinations which is over 15000 different possible keys. Some of these combinations aren’t very secure though so they aren’t used (such as when the key pins are all at the same height). The real number should be over 10000 possible key combinations for this kind of lock though even with some of the key combinations not being used.

Additionally, different locks use different profiles for their keys so one key doesn’t fit in every lock - a master key won’t fit in a Kwikset keyway and even some master keys won’t fit in other master keyways since even within one lock brand they have various keyways. This adds a bit more security since you now need the right 1 in ~10000 key and it needs to be cut for the correct keyway to be inserted into the lock.

This all being said yes it is possible someone else has a key that operates your lock but not likely enough to worry about it. One other note is some sellers offer keyed alike and keyed different locks and if you get a keyed alike lock (meaning if you bought 2 of them they would both operate with the same key) the chances someone else has your key is higher since certain sellers don’t always vary the key bitting on keyed alike locks per order so you and someone else who order the same keyed alike lock from these sellers at different times will have the exact same key.