How to count the number of people in a room with a specific sensor?
Laser tripwire

While I won't go into too much detail, it is possible to count how many people are in a room using a laser tripwire in the doorway to count how many people enter/exit. You would have to install 2 wires in each doorway if you need to know if someone entered or exited. One laser on the outer side of the doorway, and one on the inner side. For you I'd recommend a 5mW ultraviolet laser, as UV lasers are typically the safest lasers to handle, they're discrete (invisible), and photo diodes react well to them.

Double Pressure plates

this solution will take a bit of extra engineering, but if you put a mat over a pressure plate, then you can also count how many people move in and out. Something like a force sensitive resistor will be a sufficient pressure plate. The problem with this one is that people may like to stand on these pressure plates for various reasons, and it may be hard to detect who is moving where. 

Good luck!