I know we've had lower than 5 in seasons with unusual rules. I am not sure about seasons with normal rules (like this season). We've definitely had as low as five, just not sure about lower. There was one time years ago now, that AllSox claimed 5 in round one...and then claimed 5 more in round two!
EDIT: In EO XL with a cap of 255M, which had never been done before, 12 owners claimed less than 5, including je3rd with the goose egg.