PIR sensors only tell you that there is something with a different temperature than the ambient in front of them. You cannot tell how many "somethings" there and IIRC they have binary outputs, 100 people or 1 person will give the same result.
If you are talking about reading the number of people in a room in very general terms, like "there are a lot" or "there aren't many" maybe you could use a thermal camera and count the hot bits.
Rob