How many PIR Motion Sensors can I connect to my Arduino-Uno R3? I am doing a school project where I intend to measure the cafeteria line with motion sensors and the send that information with a WiFi module to a server. This is my first time using Arduino and I am not sure how many motion sensors does it support.
Most PIR sensors provide a simple relay output. That means they are nothing more than a switch as far as the Arduino is concerned.

You can connect hundreds of switches to an Arduino. With judicious use of diodes and a matrix arrangement you can hundreds with not many IO pins.  For example an 8x8 matrix of switches gives you 64 PIR sensors with just 16 IO pins.  For n pins you can have (n/2)² switches with a balanced (n/2 by n/2) matrix.