Some advertisers want to be the only one on the page and will write that right into a contract. Other advertisers will allow themselves to be around other banners if they themselves are at the top or close to the top. I offered special prices to the one at the bottom. Now that I use a program to rotate my banners for me, I only go with just a few per page. Notice on this page, too, that ad banners are different sizes.
Smaller banners allow for more per page with less intrusion.
If you do start selling ad space on your site, let me suggest that you also purchase a program that posts banners to pages on the fly. If you assign specific ad banners to specific pages, it tends to make the site look static. The CentralAd program I use can do that for you. It is my opinion that undertaking advertising is more than a hobby on the side. You must look at this as a small business.