The only fundamental limit I can think of is material dependent.
Fibers are dispersive elements, and over a transmission length L
a pulse of finite duration will spread spatially. When it
overlaps a pulse sent previously then there will be a problem.
One can, of course, multiplex the transmissions by using all the
wavelengths which the material is capable of transmitting and
then using tuned receivers to decode them, but such receivers
will each have finite bandwidths of their own, and thus the
number of such bands will be fundamentally limited.