I don’t know how many …, but it is very noisy in their flat.
(a) children they have
(b) do they have children
© children do they have

I guess (a) is correct, but [size=150]why[/size] - - - > I do not know?

Could anybody substantiate it from a scientific (linguistic) point of view?

The other two are questions (because of the verb “do”).
You don’t need a question to complete this sentence.

Thank you, Our Tort System!

Can anybody tell me under what topic this question can be discussed?

Try looking under “subordinante/dependent clauses”.

For example: … ub-clauses