Legendary is the reign of David, seen in the numerous battles he won for the Lord, including this one against the Syrians,

Then the Syrians fled before Israel; and David killed seven hundred charioteers and forty thousand horsemen of the Syrians, and struck Shobach the commander of their army, who died there (2 Samuel 10.18). 

Both 7 and 40 can have symbolic meaning in the Bible. Whether 7 has any special meaning here, it is hard to tell. David victory over Syria was complete and perfect, no doubt, because he did whatever the Lord told him to do.