About 15 I would guess. Some travellers in WTP will be upgraded to Club, but they will start with those with status, which I guess you don't have, and then those holding a reserved seat. So if you knew WTP was going to sell out it may be worth a gamble on paying for a seat reservation as a cheap(ish) way to end up in Club. So when it's a "must fly" scenario, and you don't have status, then buying seats should be considered if the cabin is getting full.