Less a case off how many people I reckon than how long between involvements... I'll explain

We have a weekly singaround/instrumental thing in our pub and somewhere between 10 and 20 participants come each week. We go round in a circle and each person has a chance to have a turn. Some people always sing solo - some sing chorus songs - the fiddlers play and encourage accompaniment etc

It probably takes an hour - hour and a bit to go round and have your own turn but during that hour most of us will have played with others or sung a chorus or whatever. Seems to work ok and people keep coming back and it grows - we have people coming specially from something up to 50 miles away this week so it can't be too bad.

I think a lot also depends on how much people keep the momentum going. Some places I have been have a dozen people there and struggle to manage two songs each in 3 hours +. We like to keep the songs and tunes rolling along. I think it also works if you have a range of types of music (unless you are a specialist place that only wants a certain type of music) and that tends to keep things rolling along.