as auxiliary verb
that when there is no auxiliary verb in an affirmative sentence
as in the present simple or past simple, it is normally necessary
to place do/does or did before the subject in questions:
saw Tom about four weeks ago. ~ When did you last see
I visit my grandparents once a month. ~ Do you visit
them when Tom is with you?
exception to this rule is when who/what/which is the subject.
Compare the following:
phoned her mum last night. ~ Who did Beth phone? ~
She phoned her mum.
Somebody phoned Beth. ~ Who phoned Beth? ~ I've no
idea. They didn't leave a message.
Which buses go to Trafalgar Square? ~ The 36 and the 21 go
to Trafalgar Square.
Which bus did you take? ~ I took the 36. The 21 wasn't
in your example, Bav, some people or how many people
is the subject of the sentence, so no inversion is needed in the
first question. When it becomes the object of the sentence, as in
the second question below, subject-auxiliary verb inversion is required:
to tell you. Some people are coming for dinner tonight. ~
How many people are coming? How many people have you invited?