There is a lot of ways that someone can interpret the saying "only bet what you can afford to lose" but people cannot really decide on how much is too much. Especially during high adrenaline events like betting.
They should set their limits beforehand. For me, they should never spend the equivalent of their salary in just one bet. You should only bet with your extra money not your entire paycheck of the month. Because if you lose, what will your plan be then? Some only takes 1%-2% of their salary but it depends on how big the salary is
I think this can only be easy for those who understand gambling, but for those who don't understand gambling, they would always want to play based on what they think about it. Most people always think gambling is all about the more you put in, the more you will win. Some people see gambling as a way of generating money, and they don't even bother to bet with all they have, thinking they can double it by winning.
As long as one is able to understand that gambling is unpredictable and winning is based on luck, it will be much easier to gamble with an amount that you can afford to lose and always stick to your limits.