I recommend many relevant broad keywords, 50 to 100. because the real thing is where ads shown not the keyword.
If you running keyword based ad, where user searches on youtube and ad will be displayed there, then few keywords are ok.
But if you want to run normal ads on all youtube. Then more keywords are better, because many users are using keywords wrongly and will be irrelevant, as the ads run, you should keep removing channels, youtube videos that are underperforming, eventually you will find top performing channels/videos.