Thy teeth. See notes on Song of Solomon 4:2, where the same comparison is used by the shepherd, except that the latter uses kezuboth, flocks, while the former uses rahel, ewes.
Related Commentaries of Song of Solomon 6:6
Song of Solomon 6:6
6 Thy teeth are as a flock of sheep which go up from the washing, whereof every one beareth twins, and there is not one barren among them.