Depends on the zone I guess but we are able to clear zones with few people. The leech zone we have cleared with 5 (geo x2 sch blm thf) and the ele zone we cleared with 6 (geo x2 sch blm thf pld). We are able to do perfedein and plouton with 4 real players playing 7 accounts (geox2 blm sch thf pld cor). The fomor zone is a bit nasty (translation not mage friendly) but we really hate it so we haven't tried very hard to low man that. We never ever bring more than 12 in on any of our runs. I would imagine with 18 competent players all of those zones are jokes.