No requirement, anyone can be a professional photographer. Its staying professional that matters.
In that context, anyone can be commercial photographer, wedding photographer, whatever he wants to call himself.
Had a rich client whose son is a fashion photographer, father got him studio, profoto lights, hasselblad medium format system, but no client (note the singular). After the boy had his fun, he can try other stuff. End of the day he is going to inherit enough that he can be whatever profession he wants (most).
Again, no requirement, anyone can be, thats why got so many goons in the industry that makes this occupation more depressing than a circus clown. Sorry for the negativity..