Interesting but what kind of load could it take ?

From what I remember the data rate with LoRa is very low...

In this case how many user could chat simultaneously? 10s, 100s, 1000s, more?

The Meshtastic site says that supports up to 80 device nodes depending on settings used.

It is more geared towards a group of trusted people communicated with each other than a general purpose network anyone can use.