Riddle I need help

100 people are in a building, each in a separate room by themselves. There is one room in the building that is empty- the only thing in the room is a light switch.
A security guard comes to every room and brings one person to the light switch room.
The last person in the light switch room has to announce that he is the 100th person. How does he know?
(They aren't aloud to communicate with Eachother other than the plan they make before the whole thing starts)