It's more a question of economic feasibility and technology than
possibility.  Uranium forms a few parts per million of Earth's crust
and can be bred into fissile material.  At most a few kilograms per
weapon is needed, so that's something like 10^15 weapons just from the
uranium in the outer 0.1% of Earth alone with current technology.

That's without considering more expensive ways to produce fissile
material, or more advanced types of weapons that might not need a
fission trigger.


The absolute upper bound is not so much likely to be a number as a
total yield determined by the mass of starship.  That's on the order
of 10 megatons TNT per tonne.  Very much more advanced p-p fusion
could approach 140 MT/tonne, if the technology could be assumed to
exist.  In theory the only upper limit to the number of "weapons"
would then be how small you can make each one (given suitable
technology), though exceptionally small ones would produce a burst of
penetrating radiation rather than macroscopic explosion.