For a fixed-wing aircraft, as far as I know, there is no limit in the amount of passengers one can carry with a commercial pilot's license. But, I think we might consider the type rating a kind of limitation.
The thing is that most aircraft that carry more than 10 passengers require a specific type rating for the pilot. That is, if you have either a private pilot or commercial pilot license - for airplane -, you are free to fly most piston aircraft. But if you want to fly something bigger, or with reaction engines, then you need the type rating - almost every turboprop or pure jet requires one, although type rating themselves don't have much to do with the aircraft's pax capacity. For example, an entry level jet like the Embraer Phenom requires a type rating, even though it can only carry four passengers (the same capacity of a Piper Saratoga).
But I don't think there is, specifically, a limit in the number of passengers a PPL or a CPL can transport. At least, I've never heard about such a thing, not for these licenses.
But there is an example in ultralight aviation. According to this wikipedia article:
http://en.wikipedia.org/wiki/Ultralight_aviation, ultralight regulation vary significantly from country to country, and it says that in the US the regs are very loose, so I don't know how this thing work there... but here in Brazil, ultralight pilots are required to have some kind of flight training. The simplest licenses one can acquire here are the CPR and CPD, that in portuguese mean Recreational Pilot and Sports Pilot License, respectively. These two licenses are the minimum required to fly an ultralight, but with either of these, you can only fly aircraft with up to 2 seats.