V.P.
We've always found 4-6 moms in a group is a pretty good number. Then, if one or two moms can't make it, you can still have playgroup. It also makes it nice that you don't have to host too often if you don't want to.
Most of the time, we didn't do any food for adults, and kids brought their own small snacks. Occasionally, we'd have a small treat for the moms if it was someone's bday.
We usually rotated houses until most or all of the kids were old enough to walk. Then we'd have playgroup at a park, mall play area, or chick-fil-a once a month and rotate homes the rest of the time.
Once all the moms started having 2nd & 3rd children, and we got to the point where we had 10+ walking kids, we stopped having it at homes, and had outings each week.
The last 10 minutes of playgroup (when we were in someone's home) were spend doing a group cleanup. Obviously, everything doesn't get put back exactly where it came from, but we really tried to make it as easy as possible for the host mom, while teaching our kids to be good guests.
Good luck!
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