Branch protection switch

In my opinion, none of this is law yet....

No, VDE are not law.

But he has to find a master craftsman who then would not install any.
After all, the master craftsman takes responsibility.

Maybe one master has planned fewer circuits.

The question is whether it is actually sensible.

I just wonder how Electrician a) comes up with 25 and Electrician b) with 18 pieces

With that number, you are not talking about fire protection switches, but circuit breakers ("fuses"). The latter costs 2€ material per piece, the former 100€ ...

No, I meant residual current devices at a unit price depending on the electrician from 100 to 150 euros.

That's why my confusion

Confusing is what that is supposed to be good for. But as others wrote, it is not mandatory. At that price, I would leave it as is, provided the fire insurance cooperates.

Just ask the electricians. At the current stage, it will probably be an estimate. One may have forgotten some, another may have been more generous. Over 20 goes quickly, for example if there is a heat pump, additional electrical devices in the kitchen, etc.

One per circuit and the things are very expensive to purchase, larger electrical box and installation.

Under the section: Haustechnik /Öko,
there is a post called Fehler Lichtbogenschutzschalter or something similar, which corresponds to the Brandschutzschalter.
The following entry to the asked question explains everything about it.