The max for these events is 19 clicks per second, or 570 click per panty. That would require 141, panties. Or, 71 times to open the chest. But you should be able to get at least 20 from the tournament. And that'll happen twice.

So you should only need about 100 panties, or about 50 times opening the chest.

I don't play too much during the day, so I'll probably just drop 300 gems on 40 panties, making this event a fucking breeze.