oh, sorry. its not a RULE as in, rulebook rule. but because of all the cards like ulairie
enquea, lom who target fellowships with more than 5 companions, there is an unspoken "rule of thumb" that you shouldn't play more than 5 companions unless you have good reason too.
like ent decks are fine with having many comps in play. some decks have protection like
shadowplay, othertimes, you just know pretty well that they aren't playing any
enquea, lom's, or maybe you just want to play all your comps and
make a run for it. etc...
basically: the RULE is no more than 9. but sometimes it can be best not to play more than 5.
hope this helped.