sure. why not stop at 5 or 6, why not go for 20? at least then, there'd be so much fighting, at least they wouldnt concentrate on just one poor fish and the aggression would be spread evenly throughout all of them.
but in all seriousness, male bettas can only grow together in the same tank under one condition: they all are siblings and their father is in the tank with them and hasnt been removed since they were born. if you cant provide the resources and conditions to make this work, i suggest you give up the idea. plus, the males will look drab as well (coloration signifies alpha dominance), so they wouldnt be as pretty to look at as you'd imagine.