Ideal group size?

Our little group now includes eight families.  We mostly know each other, but have agreed to schedule a 'playdate' to get to know one another better.  Just finding a time when we are all available is a struggle.  I'm thinking that a group of about 10 people would be ideal.  As it is, we've only had one care need go unfilled this month, and the calendar is booked with at least three childcare swaps per week till the end of October.  But I do wonder, having had a sick baby for the last four days, if a couple more people would be ideal - if nothing else but the possibility of an emergency switch of providers if need be.  But I  know MANY more people who are interested in participating in something like this, and I don't want word to get out accidentally or have people feel as though they've been left out of an 'exclusive' group.  I'm thinking it may be time to announce our success, provide a template, and let others create their own swaps.  Parents could choose to be part of one group or multiple groups, but each group would remain small.