How do I detect that there are more than n number of people in the house/room?

I am planning to put a property on a market for short-term rental and want to detect if somebody decided to use it for parties.

I am thinking about following options:

  1. Detect if there are more than n number of people in the room and the house. If for example rental is for two people and there are 10 instead. PIP sensors and m-wave sensors detect motion but not the number of people.

  2. The camera next to the entrance, NVR, object/face detection, and count the number of people coming in - this project sounds like a full-time job, plus additional equipment.

  3. A sound/noise meter to see if the sound exceeded a certain threshold. So far my quick search looking for such sensors ended up with nothing. People say to get ESP32 and the microphone but based on what I read, it is very inaccurate. There was a suggestion about Netatmo Weather Station which has a built-in sound meter, but the price is about 4x I am willing to pay, plus, I already got a weather station.

Yaah, I saw something is sold at Amazon named as ‘AirBnB/VRBO - occupancy/noise sensor’. First, it is priced at $250, then it requires a cloud service for $20/mo. Clearly no go.

If you have some other original ideas - please share.