As many as you want. You can use TubeBuddy Bulk Thumbnail Overlays to add or remove as many or as few thumbnails as needed. Note that each video has to be processed individually, and it will take a few seconds for the Bulk Thumbnail Overlays tool to do the work. Feel free to open a new browser tab and do something else while your videos are processed.
We recommend limiting bulk processing to 2,000 or fewer videos. If you have more, consider breaking them up into playlists and running the tool for each playlist separately (although kudos for creating so much content!).