Which walls are on the upper floor

I am afraid, however, that the sound insulation will not be particularly good

Sorry, but with the "signature" one should not publish such ignorance.. that unnecessarily confuses a construction layman.

You don't have to know everything.. but then you should simply refrain from giving such answers!

Thank you very much for the answers...

How thick should the walls be for good sound insulation??

The developer now wants to use 10 or 12 cm walls... is that enough??

That simply depends on your requirements..?
If you have increased sensitivity... then the wall must be adjusted accordingly!

You can also see how this looks on the websites of companies like: Knauf, Rigips, or Fermacell.. go there to the button "Schalldämmung" and you will see the wall variants offered there with the corresponding values. It is also clearly defined what requirements each company has so that their respective calculation is fulfilled... meaning the sound insulation value is achieved!
And you can then give this exact requirement to the general contractor.