Simplest thing I can think of would be to use two trip beams to see people passing in front, and depending which one is first, see which way they are going like a rotary encoder. But this would require that only one person fit through the entry way at a time.
A PIR sensor is not going to count people well, even if you have more than one and set it up well. They trip whenever there is any change so you never know if it will trip just when someone walks by, or if it will trip again as they leave the range or if it will untrip/retrip while they are still in the range.
The conditions by which people will be able to enter/leave will be very crucial to this project if you want to accomplish it without video recognition.