The editor creates ads for every selected combination: each ad set in each campaign, in each account, receives all the creatives you upload (each media file becomes one ad in every ad set). For example, 3 videos published to 2 ad sets generate 6 ads in total. To select several options, hold Ctrl (Windows) or Cmd (Mac) while clicking. In the results list, each row shows the name of the ad set where the ad was created, so you can see exactly where each one landed. Check the planned total shown in the progress bar before confirming large volumes.