A lot of them have similar names and their ID numbers are close together. It can't just be a coincidence that almost everyone has a username that is either a basketball player, cartoon character or pro wrestler. Even if it is allowed, it's a bit disconcerting that so few people can monopolize the competition.
Yes, they explicitly stated that you can use multiple accounts in Number Hunt competition.
You can only win 1 prize per account, but you can use multiple accounts if you wish!