Each ad network will be run a little differently, and will have different variables that you'll need to be aware of. One of these variables is limits. These limits can be set in your ad server, or possibly within the dashboard of the ad network itself.
It's important to understand limits to properly budget and estimate revenue. Limits basically put caps on how many ads can be shown. One kind of limit is a daily impression limit, which means that you have to cap the number of ads you can show on your site in a given day. Another limit that you'll likely have to deal with is called a “frequency cap,” which means that each visitor can see the same ad in a given timeframe (usually a day) only a certain number of times before that ad isn't shown to him or her any longer. This can make a big difference, because a frequency cap of two per day, for example, means that each viewer may see only a handful of ads before they're no longer bringing in revenue for you. To compensate, you'll need more traffic, more ad networks providing ads, or both.