Here is a page on bungie.net that explains the matchmaking playlists and gives the max party size.
Play List Information So we are limited to a party of 5 for Big Team Battles but we can take a party of 8 in for a Major Clan match. We should get some clan matches going. If it's as simple at the other match making we should be able to have 5 or 6 matchs a might, more if we have two differnt parties going.