Is there a technology for collecting data on the number of people in the room?

Question

Data processing

Hello.
The question is: is there a technology that helps collect data on the number of people in a room? That is, so that I could receive real-time information about how many people entered and left the premises.
I know about iBeacon and others, but I would like something with a lower entry threshold.

Answer 1

Simple pinwheel input and output one turn +1 or -1

Answer 2

OpenCV + noise level + temperature + input turntable)

Answer 3

What about using a webcam? Similar to openCV