I finally got an answer, which of course is derived from a model (inspired by Engineering).

But at least here is an answer, which I could not get before, and I got it reading the Neural Engineering book.

Basically an estimate of the number of neurons is a function of:
- the noise leve of the neurons
- the required precision of the representation
- the required range of the representation

There is also a formula, which is derived using calculus.

Now my next question is: would it be possible to estimate algorithmically how many neurons would be needed for a certain number of typical everyday representations?

Obviously the final question would be: Where can I buy those neurons? :-)