Difference between "what" and "which"

Good evening,

I recently started to refresh and improve my English and bought two grammar books. I learned that there is a difference in using "what" and "which" in questions. You use "which" for small numbers of possibilities and "what" for more general questions. In the second book there was a sencence in an excercise that confused me: "You know that Liz reads a newspaper every day. You want to know which one. Ask her." The correct answer is: "Which newspaper do you read every day?"

So why do they use "which" instead of "what"? There is no small number to make a choice.

I'm looking forward to you answer.

Nice greets,
Alissa