Below I looked at famous and respected magazines that my magazine would sit along side on the shelf to see how many pages they had. Looking at the amount of pages they have gives me an indication of the amount of content they contain.
add them together
and divide by 15 to get the mean amount of pages in a magazine...
With many of the magazines I looked at there was a vast array of advertisements, which I wouldn't include in my magazine so I will go for a round number of about 100 pages for my magazine.