[susan53]

Which is usually used for a closed question - ie one with a limited range of answers :Which football team do you prefer - Manchester United or Arsenal? - whereas what is used for an open question : What is your favourite football team?

However, what is often used instead of which in closed questions too (What pen do you want - the red one or the blue one? )

When there's a large number involved, but that number is actually limited - eg the pages in a book - the speaker can see it both ways : as limited because she knows there are 125 or as open because there are too many to be bothered about counting. So either which or what may be used. It comes back to the idea that grammar is meaning - what you say depends on what you choose to express, or "mean".