"I like number 2 and 3" or "I like numbers 2 and 3"?

What is the correct form and what is the exact name of this phenomenon. I tried looking it up myself but I could only find a German explanation that might also explain the English problem.

An example could be:

Please read page 1 and 2

vs

Please read pages 1 and 2

or

I like number 2 and 3

vs

I like numbers 2 and 3

and another one:

I managed to solve question 6, 7, 8 and 9

vs

I managed to solve questions 6, 7, 8 and 9

I believe that you are noticing something called ellipsis. The correct way to say these would be

Do problems 1 and 2.
Do problem 1 and problem 2.

However, you can shorten "problem 1 and problem 2" to leave out the second instance of the repeated word "problem", getting:

Do problem 1 and 2.

There are only some cases in which you can do this. I would say this case is borderline.

This has nothing to do with the German link, which is talking about constructions like

Macaroni and cheese is my favorite meal.
Thunder and lightning is scary.

where two paired words which occur together often are treated as a singular construct.

If by “+” you mean “and”, then you want the plural. E.g.:

Do problems 2 and 3 from chapters 4 and 5 on pages 127 and 128.

It’s either Please read page 1 and page 2 or Please read pages 1 and 2 and either I like number 2 and number 3 or I like numbers 2 and 3.

In your last example it’s either I managed to solve question 6, question 7, question 8 and question 9 or I managed to solve questions 6, 7, 8 and 9.