Heat differential is almost impossible to track in a residential situation as a means of counting people, although you can use passing changes in heat which is how most “Motion” sensors work.
Heat differential works great in a totally controlled environment, like a museum vault. The problem with a typical Home is that there’s so much variation even in an empty room. There’s often as much as a 15° difference just from the area right next to an outside window to the area closest to the furnace vent. And the hotspots in a room will change over the course of the day. When the furnace is on, the air out of a forced air vent is typically between 135 and 145°F. And there are other heat sources like incandescent lights, stoves, fireplaces, room heaters and some other equipment. Even a dog. You put all of that together, and you just can’t count people based on the heat of a room.
The most practical method is a trip beam, well, two actually at the entrance to the room. Some people have done that, and one of the micro location threads has a link to an actual project plan for it. With those you can literally count the number of people who enter the room and then the number of people who leave. They’re very reliable, they work very well, they just cost quite a bit and the install is a little complicated. But you don’t have to have people carrying anything with them, and you don’t have to keep recalibrating for summer days versus winter nights. So this technology exists, it just costs more than most people want to spend.
Beyond that, again as discussed in the other threads, micro location is something that everybody wants, but practically we don’t have yet. The best guess for five years from now is that will have technology that can recognize heartbeats. But that’s not available yet. 
There are some community members that have used the “wasp in a box” model with some success. You might find that interesting.