"Probably theoretical mathematicians can come up with a theory to prove anything and because no one else understands it, it can't be disproven." That's not how science works. There are many curious mathematical "systems" that do not correspond to any known physical reality -- as so far known, that is. Non-Euclidean geometry, for example, was "invented" and investigated before it was applied to relativity. Those theoretical systems will remain on the books and be just curiosities until such time as they are found to be models of some physical system, if ever. (By physical, I mean anything apparently real, including biological, chemical, psychological, economic, sociological, political, etc etc) It often happens that there are multiple mathematical models for the same physical reality, and then the real work of the science community begins by performing experiments whose results should be either consistent with or inconsistent with one or more of the "competing" mathematical models. That is the situation with String Theory at this time.
"If you can mathematically prove more than 3 dimensions, then why would it be limited to 11? Why 11 and not 7?" String Theory does not prove that more than three dimensions exist. What is does is present a set of equations that describe a conceptual reality that accounts for both relativity and quantum mechanics. And, both relativity and quantum mechanics have been tested extensively for consistency (or otherwise) with measurements, and they have both passed all such tests. (Well, up to very recently) Now, it is the nature of those equations, which include a variable N to represent the number of dimensions in that conceptual reality, that determines what numerical values N might have in order for meaningful results to arise. The value of 7, for example, produces results in which the left and right hand sides of certain equations have different values.