An entire fleet the Borg were prepared for, thanks to Locutus. And because Starfleet didn't know squat about how to fight them and this was their first major engagement with them. And most of those ships being older vessels to begin with.
I mean heck, the original script for "First Contact" had there being a fleet of Borg ships engaging a Starfleet armada, not one Cube. With most of the Borg fleet being destroyed in normal combat.