FAQ for
The Matrix Revolutions (2003) More at IMDbPro »

Having "infected" the Oracle, Smith gained her powers. Using her prophetic ability, he foresaw the end of the fight and which version of himself was there. If that is the version that will defeat Neo (so he thought), then it is the only version that need fight at all.

There is another school of thought that Smith, as Neo's opposite number in the equation balancing itself out, could not fight Neo to anything but a stalemate, regardless of how many Smiths joined the fight. We see this demonstrated to a lesser degree during their brawl in The Matrix Reloaded. It wasn't until Neo chose to flee or surrender that their fights ended. Because Smith couldn't see past any decision he couldn't understand (Neo's decision to surrender), he did not foresee his ultimate destruction once he defeated Neo.