Did you think about how you want to RELIABLY count the number of visitors in a room? Where do you get your counter signal from?
I did it once with a IR transmitter and two IR receivers TSOP, long time ago... You don't need a microcontroller for it, you can use a counter IC too.
Or do the visitors have to pass some kind of baffle gate?
I wanted to achieve something slightly different, look at THIS thread...