ESP8266 / ESP32 - Health issues
Ok, that system makes no sense at all. IEEE802.11a/g/n as used by these devices is collision avoidance-based. That many devices in this little space would inherently lead to congestion.


  Is it healthy to have so many active devices in such a small space?


No, because doing this means you've probably been drinking heavily!


  How many wifi devices (ESP32 or ESP8266) can be “mounted” in one square meter (Ideally some researches or legal norms, not “just experience” or “commons sense” because this project aims to be a commercial one)


That's not touched specifically by legal limits, because the single-device legal limit suffices: these devices will avoid interrupting each other, so there will be constant congestion, and very few devices (as in: 1, or 2 per channel pair) will be active at a time.

So, already covered. Also, this commercial project will be a total failure.

There's nothing unhealthy about this. These devices are specified to not be active at the same time, so a) your network won't work, and b) even if it did, it would be totally harmless.

Also note that wifi devices do power control, i.e. when they talk to another device they know is close, they reduce their output power.

Just a quick calculation: 150 devices on 8 m² is one device every 12.3 cm, if you used the densest possible packing (that's hexagonal packing in the 2D plane). At that distance, you're not even in the far field of the antennas with your neighbor's antenna, and thus, the thing will totally break down, I'd presume.