An online system presents advertisements and content items to its users in a feed of content items (e.g., a newsfeed). The online system enforces one or more advertisement policies regulating insertion of advertisements into the feed and determines a predicted likelihood that enforcing the advertising policies will prevent insertion of additional advertisements into the feed of content items when a request to present content via the feed is received from a user of the online system. Advertising policies describe conditions preventing insertion of additional advertisements into the feed (e.g., positions in the feed that may not be occupied by advertisements, a minimum distance separating advertisements in the feed, etc.). Based on the predicted likelihood, the online system determines whether to request one or more additional advertisements for insertion into the feed from an advertisement service.