The one Borg cube they sent to earth was enough to surely defeat the armada. The only reason the Borg ship was destroyed was because Captain Picard’s interference. 

Picard was assimilated by the Borg therefore the Borg gained a lot of experience about the Federation which resulted wolf 359 but when Picard’s humanity returned and he was no longer a Borg drone we had a lot experience about them which there was a lot from the Collective hive mind. This gave Picard the advantage of knowing weaknesses against the Borg cube. If Picard did not aid the Federation armada against the Borg heading to earth, the Federation would have been swiftly defeated.

The Borg objective is to assimilate anything worthy for assimilation and improve Borg technology; therefore most entire Borg cubes across the galaxies  are assimilating species,they also are defending there own space and above all protect queen complex but for those little confused by that can refer to it as Borg capital which you can see protected by Borg cubes on one the voyager series

Sending more than one Borg cube would logically be a waste of resources—overkill—and the Borg are overconfident that they will succeed in assimilation and as can be imagined they can spare Borg cube to it be very meaningless to the queen unless losing more than one cube perhaps, and as matter of fact the Borg cube wouldn’t have failed if wasnt for Picard’s knowledge of the Borg, which we used against them just as they used knowledge they assimilated from him.