story purposes, i dont think theres a canon reason why they only sent one. the producers probably couldnt think of 2 cubes vs a bunch of way less powerful ships. i agree that maybe 1 or 2 spheres would have filled the gap a bit.
There are a couple of reasons, possibly; first simply being ego, the Borg aren't going to overdo it, and the way they see it, one ship could easily win (and it very nearly did).
Secondly, it could be because they couldn't spare any more. I think Frst Contact was around the time they were at war with Species 8472.