My party worked with fewer people. I checked it on two playthroughs, first you need to buy a lot of slaves + you must have a mistress. At some point an event will appear and parties will be available. And then you can carry them out with any quantity. I had it available even for one slaveanyone managed to trigger the slave party? tried multiple times now different walkthroughs and all end up the same got the dialogue with the mistress to activate it but can't trigger the party no button to start it nothing, searched the thread someone mentioned that you need 10 girls and 10 guys, i had the basement full with more than that even still nothing guest party works fine