I can't imagine what the justification could possibly be for a limit on the number of downstream receptacles. In the commercial world you have a restriction on the total number on a branch circuit (the 180va deal) but that doesn't address whether they are upstream or downstream of a GFCI. Beyond that it only depends on the feedthrough ampacity of the GFCI. You can exceed that with one duplex as easily as 10. (2 hair dryers or toasters?)
The only other thing I can possibly think of is the "antenna effect" of a long conductor (false trips) but again, that can just be one receptacle way out in the yard.