It would be nice if we could set impressions limit to a banner, for example, a client pays me 1000$ a month, and I guarantee him 700,000 impresions monthly. What if he asks to have two banners in the same placement, in the same time? If we had impressions limit, we could set 350k limit to two banners, and not give him more impressions that he paid for.
The AdPlugg Business Plan has that functionality. The Business Plan includes Delivery Controls that allow you to set a certain number of impressions for each ad per month. It also includes pacing controls that allow you to space the impressions out over the course of the month. You can switch to the Business Plan by clicking your username in the top right and then going to Account Settings > Change Plans.