The GeoTIFF specification addresses only the georeferencing and
geocoding aspects of TIFF images so it says nothing about the number of
bands in the image. The band limit will be inherited from the TIFF 6.0
specification, which stores the SamplesPerPixel information as a signed
short. Thus you are limited to 32767 bands. Note that any samples above
and beyond those implied by the PhotometricInterpretation assigned to
the image are not required to be read by a Baseline TIFF Reader.