So, basically, 18 people have to choose 17 "chairs", or numbers. Their goal is to pick numbers which no one else would pick. 

If they pick the same chair as someone else, then I use an RNG to decide who wins the seat. 

So, how efficient will they be? How many seats will be left open?

Resolves YES to the number of chairs left open.

If people don't participate, then I will deduct the number of chairs left open by the number of people not participating.

Here are some previous results from my last 2 rounds of games: