How many possible combinations are in a 5 digit lock?

That depends on what you are talking about. A digital lock of five numbers has 5 x 4 x 3 x 2 x 1 = 120 combinations, but a keyed lock with five tumblers can not use all combinations because the part near the fingers must have enough metal to be strong enough to turn the lock. If you have a box of random keys, probably about one out of ten will open a car door, even though it does not resemble the key for that car.