How much mathematics and what level should commercial pilots have?
So there are potentially two ways to interpret your question. First is a pilot who holds an FAA commercial pilot certification, second is a societal occupation label generally referring to a pilot performing as a crewmember of a 14CFR Part 121 scheduled air carrier airway, or colloquially, an airline pilot.

In both cases the idea that any pilot need to be exceptionally capable at math is a fallacy. I suspect it's a holdover from the 40's and 50's when all flight planning calculations needed to be done by hand. Now all of that is automated and even allowed during the FAA written tests. Additionally, needing strong math skills is often claimed when an individual is actually thinking of a test pilot or aeronautical engineer, which both require very strong algebra and calculus skills. 

That said, there are some areas in the higher level written exams that rewrite you know how to calculate weight and balance, but these all relay on standardized some equations that aren't exceptionally difficult for even a novice. These written exams include the Airline Transport Pilot (which will be required for the second interpretation), Dispatcher Certificate, and Flight Engineer.