How long can you keep counting "threesome", "foursome", "pentasome"? And when does it become an orgy?
Looking at Wikipedia's list of Numerical prefixes, quattuorvigint sounds challenging Sexagintisome becomes too obvious again, and millisome sounds a bit sad. That leaves us with infinisome or just plurisome. I should probably start sleeping already to gain strength.

It always reminds me of the crook in Burn Notice, when he was about to die, he said with a sad voice: "I've never had a foursome!".
Allow me to use Richard Branson's famous airline quote out of context: the easiest way to get a foursome is to start with a pentasome and wait until someone falls asleep