count question

Hi again I was just wondering if I was doing a query and was using the count function i.e. count(people) "number"
and I only wanted to display number if it was greater than 20 how do I do this?

I have tried number > 20 but this brings up an error message that says the column doesn't exist.

Thanks in advance.

having count(people) > 20

Thanks for your help. :)