The major issue I see with setting a cap on advertiser's is that it would be super easy to grief. Say it's 2 advertiser's and any extra get chat restricted (as an example). As soon as the first one goes up someone else unrelated to the venue could make a second. Now the actual second advertiser's will be punished since someone else got to it first.
Not saying this would be common but any system that can be exploited by bad people isn't good.

I like the idea apkallu had about collapsing the messages. Take it a step further by combining multiposts as well (3 messages ads).