When creating or editing a line item, you have the option to set how many of its creatives can display on a given page. You can change its delivery settings to any of the following, under the Display creatives menu:
As many as possible: Select this option only if you'd like to enable roadblocking (serving several creatives form a line item together on the same page). Learn more
Only one: Only one creative from this line item can serve to a user on a single page load. Once the ad server has served that creative, it won't serve any other creatives from this line item. This option is useful if your creatives are similar to one another.
One or more: Any number of creatives can serve to a user on a single page load: just one, just a subset of the creatives, or all creatives.