Should I add my business experience in my application of master program in math?
[Context: I am a CS lecturer in the target country] I really see no reason to hide an achievement of this nature, even if it is not tightly related to your application field. It shows that you are capable of committing to a long-term project with responsibilities. Perhaps it will not give you a significant edge over other candidates at this stade (application in master 1), but in the future, researchers interested in choosing you for an internship will certainly find this information useful.

It does not harm to mention this experience, and it might be useful now or later. Also, if you don't mention it and someone finds out, they will wonder why you wanted to conceal it. Really, I can't see a valid reason to do so (except maybe if what you founded was an escort agency or some other adult business, and you wanted to keep that for yourself).