If I don't believe that I can win millions from gambling, I wouldn't be gambling. There's a difference between hoping to win and being so desperate to win. You have to understand that gambling needs luck, no matter how good you are, you still need a bit of luck to win in gambling.
The amount a person should be willing to risk shouldn't be the question, the question is how much is that amount you're willing to risk worth to you. $100 may mean nothing to me person and that same $100 may mean a lot to another person. You can't take an amount that's so much more that you can afford to lose to gamble with the hope that you'll win the bet. It ends up badly most of the time.
When people become so desperate to win or keep trying to win back their losses, they end up being addicted to gambling and that is a bad place to be.