Another guesstimate:
Upper bound for the number of Medchem-based molecules:

8000 fragments (CFRAGs and FRAGs)
8-fragment molecules (more or less)

8000^8 / 8! = 4.2e26

 (+) However, [3-7,9-]-fragment molecules should be counted.
(++) However, multiple ways of connecting given fragments.
 (-) However, not all molecules fit.