With 8 Modules i've no problem. 
Hello Dezider Buch,
WELCOME TO THE FORUM
From functional side of view it should work. I don't think that the WIFI should cause any problem.
BUT:
When you use a lot of Shelly 2.5 in Dual-holders AND put them side by side then you can get temperature problems. For Shelly 2.5 I would recommend to let at least 1/2 unit space between the holders. Additional the ambient temperature has an influence to the resulting inside temperature of the Shellys.
hello, thank you Guzzi-Charlie for advice about temperature problems I have big cabinet so any problems with space between modules. Maybe I can use for safety external temperature sensor for shely 1pm and I become push up warning notification that temperature is above 40°C in cabinet. Or maybe use some of billig tuya WIFI thermostat for this message.
I agree to have at least 1/2 unit spacing. Btw you will need additional terminals for power distribution. It is very easy to get messed up with all the necessary wires.
Don‘t forget not to mix up phases for control and power at a single device, otherwise you‘ll damage it.
Each single Shelly will tell you the internal temperature, therefore additional sensors are not absolutely necessary, maybe helpful.
You will need beside all the Shellys a central controller. Without your possibilities (i.e scenes) are limited. Which controller? There are so many possibilities. Software based or hardware (PLC), it‘s your choice. You could use instead (or beside) the Shelly APP and/or Shellys cloud services, reliability is another question. I would never trust the cloud.
