How many pages? Why, however many pages you need to get the intent of your content across. Or, is the intent of your content just to put ads on them? Why do I smell MFA?
There is no limitations. I suggest you should put adsense on pages that has maximum bounce rate and maximum traffic.
I have thre sites with over 800 pages each they all have adsense on it except pages like my contact pages, privacy pages, affilates page, links page.
Well, I got my adsense approved when I had more than 15 blogposts. So, I think you need a lil bit like more than 15 pages or posts. - Digital -
Its ok if you have one good page. But you will need another page for privacy policy. So 2 pages minimum.
At least 1! Ideally with a minimum of traffic. Some years ago it was 100 views if my memory is good. Your number of impressions is the important factor.