iMovie doesn't have a live counter feature. I searched around the internet a bit and couldn't find an app that provided an on screen click tracker counter. You can search around yourself and see if you can come up with one.
However, you always could make one yourself by recording a voiceover while tapping on your desk top in the time intervals that you want. Or you could buy a metronome and record its clicks. I think that's what those You Tube videos do. I understand that drummers sometime use these click trackers when they practice, so you might search metronomes or drummers, and the like.
-- Rich