I'm not sure of the terminology. In the US we have open mics, song circles, music circles with both singers and instrumentalists (tunes and songs), jams, and "hoots" where most anything goes.

Just two are fine if the other person is compatible with your music. On the other hand, many more than two dozen can mean only one song for each in the evening. What I like best is the "jam" where instrumentalists if they are good can join in and back up the singer, and where all the singers can join in on the chorus if appropriate (but I would hate to have to do only chorus songs).

Going around a large circle, where everyone takes a turn (some are prepared but others not) can sometimes be a drag. The anarchy alternative works well with a good host who makes sure everyone who wants to sing or play gets a chance. It can be more interesting since one musician can take off from another's music. In open mics you need to have some procedure to set the order (order of sign up, random order, semi-random order (MC makes choices), etc.)

I would tend to go with Mary's (Guest-mg) suggestion, but then that's not considered democratic, and we have to be democratic don't we?

Cheers, S. in Seattle