Rule of 6
The Rule of 6 is an informal game term. There is a soft limit of less than six Free Peoples companions. This soft limit isn't set by the rules, but rather the existence of Shadow cards like Úlairë Enquëa, Lieutenant of Morgul (1U231) which harshly punish Fellowships of six or more characters. "Shotgun Enquea" and other similar cards are a strong incentive to never have more than five companions out at a time.
Unlike the [[Rule of 4] and Rule of 9, this isn't a hard and fast game rule. If you can weather the setbacks from these Shadow cards, you can simply carry on with a large Fellowship. Also, while most Shadow cards of this sort target six or more companions, some instead spot five or more (e.g. Savagery to Match Their Numbers (1R139) ) and a few spot seven or more (e.g. The Number Must Be Few (1U260) ).