First: Normally, a single Borg cube is powerful enough to assimilate or wipe out an entire planet. One cube made mincemeat of a fleet of 40 starships at Wolf 359. A cube is as powerful as a whole armada; it's just that the Borg are a collective society by nature, so they put their whole armada in a single box, literally. One cube would've been more than sufficient to assimilate Earth if the Enterprise hadn't had the benefit of Picard's inside knowledge to find its weak spot (in both BOBW and FC).
Second: The Federation is not the center of the universe. From the Borg's perspective, it's a remote, distant society on the far fringes of the galaxy. As the Borg expand through the Delta Quadrant and beyond, they're no doubt constantly warring against dozens of other civilizations on their borders. Presumably it's those immediate enemies that take priority in their planning and use up most of their resources -- as we saw in VGR with the Borg's war against Species 8472. A much more remote civilization like the Federation might be worth investigating considering its level of advancement, but as far as the Collective's overall priorities go, the Federation would be pretty low on the list. So it wouldn't be worth sending more than one cube, since normally that would be more than enough. If that cube got defeated, they could just send more when it was convenient. It wasn't until Voyager ended up in Borg territory and started to inconvenience them more directly that the Collective started to see the Federation as a more serious threat -- and any large-scale invasion plans were scuttled when Voyager destroyed their transwarp hub in "Endgame." (Although both the Destiny novel trilogy and the Hive comic-book miniseries show the Borg launching massive invasions of the Federation some years after "Endgame," though only Destiny is explicit about it being in retaliation for the hub's destruction, which elevated the UFP from a nuisance to a major threat.)