Why is it have to be 8 or more rules about money?

Why limit the rules of money? Money was created by human and of cause the rules can be more and not limit to certain numbers. Money can evolve just like any living matter. I got piss-off really whenever I heard or saw an article that says 8 rules of money. These often called guru or expert in financial, frequently telling people when dealing about money do not limit our thinking of the many ways we can earned money. Yet, when they tell people about money these guys always specified the number i.e. eight, ten, twelve and so on. In my opinion it should not. And furthermore these guys also always picked the number 8. Why? Just because number 8 means luck or good fortune, it does not mean you have to use 8. It contradicts with their philosophy.