Max number of Shelly 4PM on Ethernet network ?

Thats more depending on your Network. 20 Shelly pro shouldn‘t be any Problem.

Hi Jean-Marc , welcome to the forum.

As Devil already wrote, there is no problem with the Shelly. >120 pieces are running here in my network.

From a network perspective, it is not the data of the end participants in the network that is important to ensure good data traffic.

It is important to use good quality switches. The larger memory for Mac addresses, the better is.

And of course your switches need enough ports.