The amount of ad spaces you can use varies from company to company. Google limits you to 3 ad spaces on one page. Some companies like bidvertiser and oxado don't place a limit but in general it's best not to use too many ads because otherwise it may annoy the users visiting the site...
your webpage takes for ever to load and alot of sites tell you that webpages can not be used to only display ads.
some CPM companies allow having more than 3 ads on page but each ad must be different size for example you may have 1 leaderboard + 1 Banner Ad + 1 SkyScraper Ad +1 Square Ad this is 4 on one page