I think spb has covered most of the possible scenarios, but as I have posted elsewhere on another forum, 3 of the local folk clubs I go to work very much on an ad hoc principal, in that the MC for the night just asks people who turn up on the night if they want to do a floor spot, which will, in most cases, just be ONE song or tune, so you may get 3 or 4 each half - certainly no longer than 15 minutes of floor spots each half all told. These stalwart MCs seem to have good memories, so that if you go regularly, and don't get a floor spot this month or week, your turn will probably come around next month or week. And if somebody really famous or good happens to drop by one night, and your wonderful MC does not recognise them, you can always "tip the wink" to the MC (who doesn't know everyone in the whole folk universe) - "that's the right honorable Mr(s) excellent folksinger/player - why don't you ask him (her) to do a slot?"
And as for trying to pack every person known to have ever done a floor spot into one night - forget it - we agreed this at our local club: just decide how many you're going to have and stick with it. The local regulars all understand this policy, and no-one feels offended, even if they have turned up prepared with guitar or whatever else, that they don't get asked that particular night, and nobody considers it to be presumptuous to turn up with an instrument - better than having to hike back to your distant car if you DO later get asked to do something!