They will be adding more Difficult NMs down the line. The current NM's aren't made to be super difficult or need too many people. Look at the details for faction NMs for example. *Recommended for groups of 3 or more* is what it says in them. not 8, not 15, just 3. the HARD HNM's may require you have the full party group buff, there may be raid groups that allow more than 8 players in it later, and RNM's that require like 16 players in those raids (they are adding Raid Zones/Dungeons).