The WRSTC sets the standards which PADI, and it's other members, follow. There is not a mandatory number of days, it is set by how much diving can safely take place in a day.
Sounds pretty much like mandatory minimums to me but if you want to quibble that is fine. And, important to note that WRSTC does not set the standards for every single PADI course, they only have guidance posted for the major classes. PADI does have some latitude with their own course standards, especially for specialties.