Couple things now that I have read all this again.
1. We still are in the midst of the current exchange and about 2/3 of the folks have sent there postcards. So we're still awaiting some more. So, this means that as the numbers grow, the waiting grows too as particpants finish their card making and posting.
2. I'm not sure I like the idea of 'cutting off' the sign up to a limited number of people. I'd rather have two or three groups than limit the number of people.
So all that being said, maybe this question is premature. Let's wait until this round is 'substantially complete' - that is when say 90% of the particpants have sent their postcards.
Then, let's sign up people and see how many we. Seems like if we have 50 sign ups, people are willing to send postcards to everyone. If we get more, then let's ask this question amongst the particpants to see if we need to split into two groups.