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.
There are ways you can use multiple accounts and there are ways you can't. For example one big no-no is using multiple accounts to max bet on a game. Let's say there is a max bet of 1 bitcoin on a game, and you have 10 bitcoins you want to wager, you can't just open 10 accounts and wager 1 bitcoin on all of them. That is not allowed and that will get you banned and moreover you will not only get the wins and all of the bets will be voided but you will also lose that 10 bitcoin you played with as well.
However this is just a fun thing, there is really not that much to really give up, casino already gives away something so they are not going to lose if you try just once or 100 times since they are not going to care who wins at the end, as long as they are giving it away, they are doing their part.