I don't think PIR will allow you to do this, my suggestion is to use openTSPS, it's an open source software that does video recognition, so you just use a webcam and it will do all the processing including counting number of person in front of the webcam. OpenTSPS uses OSC to communicate with other software, so you can use Processing to get this OSC value and transmit it to Arduino via serial or use the Firmata firmware so you can just do all the programming inside Processing. Seems to me that this is the easiest method. You'll need a computer for the installation though.