Where did the number 48 comes from?
Most speculation I've seen centres around the idea that one ship survived, since Hanson mentioned 'forty ships' - although this could well have been a case of giving a rough estimate. He mentioned Klingon support so they could conceivably have been involved.
Ships speculated to have survived were either the Ahwahnee or the Endeavour. Ahwahnee because it was referenced later - but with a different registry. Endeavour on an unconnected reference in Voyager about a Borg encounter.
We saw about 20 survivors in Saratoga's pod that presumably made it. A couple made it back to the Delta Quadrant as Borg drones. Other than that - it's speculation with no basis.