We don’t limit how many times a single reader might see the same ad, but we do limit how often they can see the same ad. Our auction will not serve a unique ad to a reader more than once every 3 days.
Please note that if you are running multiple ad campaigns with the same ad image (e.g. if you are targeting different audiences but using the same image for each campaign), a single reader may see that image more frequently than every 3 days if they fall into the audience of two or more of your campaigns.