How to select a home automatic circuit breaker?
As a rule, each CB must be dimensioned to protect the smallest unprotected wire in its circuit. In your schematic, the first CB has to protect the 4mm2 wire going to Room 1, but also 2mm2 wires going to outlet 1, outlet 2 etc., because those wires have no individual protection. Because of this, CB1 should be dimensioned to 15A or less. Otherwise, a 20A load on outlet 1 will not trip the protection, and the 2mm2 wires will overheat and catch fire.

The same reasoning has to be applied to circuits in Room 2 - Room 5.

The main CB is there to protect the wire of electrical company and not your house, since all the wiring inside your house is already protected by individual CBs. Therefore, the main CB is not supposed to be dimensioned to the sum of your individual CBs. In my experience it has to be dimensioned (and sometimes even installed) by the electrical company, and you don't have the right to touch it afterwards. Also in my case the price of electricity depends on the main CB setting: if you want more peak current, you have to pay more.