From my xprience,and as I've read we need
healers:2 holy paladin(3 if not any retri),1-2 holy priests,3 shaman,1-2 druid
dps: 1(or 2) shadow priest,4mages(one frost),3 locks,2 hunters,1 elemental shaman,2 rogues,2 warrior,enha shaman
tanks: warrior,in some bosses paladins for ot's,and druid.I don't remember if we need 1 more tank instead of 1 dps.
Also I suggest to focus in 1 raid,to start progress not,2 or 3 different(like tk,or outdoor raids).
What is your opinion?