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.
well that would have been a joke.
You have to remember the context with which the Borg were introduced.
to quote Q:
You judge yourselves against the pitiful adversaries you've encountered so far - the Romulans, the Klingons. They're nothing compared to what's waiting
You can't outrun them, you can't destroy them. If you damage them, the essence of what they are remains. They regenerate and keep coming. Eventually you will weaken. Your reserves will be gone. They are relentless!
The Borg were NOT introduced as some villain of the week, the whole purpose of them was to be something beyond what starfleet could realistically handle. Sure they could scrape an even numbers encounter with the romulans, barely, but the borg were supposed to be LEVELS above that. They have massive, technologically advanced ships, the collective consciousness and experience of millions. They can adapt to you a hundred times as fast as you can think up new ways to combat them, The Borg were in essence an unstoppable force.
For Voyager hardcore fans, they don't want to admit the show got some things wrong, so they instead point the finger at TNG saying that the borg in TNG were "silly" and "overpowered" and "unrealistic" I myself am a fan of the show but I am resoloute, voyager watered down the borg, badly.
In the end, they were no more of a threat than " the pitiful adversaries you've encountered so far - the Romulans, the Klingons" They lost their edge, and it was illogical writing that did it.