It's not made clear and the script doesn't say.
In TNG : I, Borg, Hugh's body was reclaimed by a Borg "rescue vessel". There's no immediate evidence that his individuality spread any farther than that ship (into the wider Collective) and the crew complement of a rescue vessel isn't canonically confirmed.
We actually see 27 individual borg in the two episodes plus there must be at least one on the ship that attacks the Enterprise. This sets the lower bound at at least 28, plus the number that have died (see Hugh's statements below).
A Borg 'Rescue' vessel would likely have no more than a few hundred drones as crew (conjecture)
Based on the size of the hall on the surface and the size of the ship in orbit, those could comfortably accomodate a couple of hundred drones (conjecture)
On top of that, there's been some wastage.
The Borg have lost the ability (and desire) to assimilate their victims. Any casualties are unlikely to have been replaced.
Hugh says that "many" of the drones on his ship died of starvation and infighting.
Lore has apparently been experimenting on the remaining Borg, killing several in the process.
My guess - no more than 100.