Every 30 seconds would be a bit much don't you think?
I'm not sure whether YT actually limits it, though you can set ad breaks in which an ad might play if your video is over 10 minutes long. As far as I'm aware YouTube usually only plays ads every 7 minutes over the entire platform.
I would agree that having ads every 30 seconds is a bit much, as that would also interrupt the flow of your video. I also haven't heard YouTube imposing a limit on the amount of ads you can place in a video but it doesn't mean they will always show.