The bell boy solved this problem.
He asked the thirteenth guest to wait a little with the first guest in room number 1.
So in the first room there were two people.
The bellboy took the third guest to room number 2, the fourth to number 3, ..., and the twelfth guest to room number 11.
Then he returned to room number 1 and took the thirteenth guest to room number 12, still vacant.
How can everybody have his own room?