Answer to the question:

How many Beings were present when the law was proclaimed on Mount Sinai?

Answer. Only two Beings. The Father and Son.

"When the law was spoken, the Lord, the Creator of heaven and earth, stood by the side of His Son, enshrouded in the fire and the smoke on the mount. It was not here that the law was first given; but it was proclaimed, that the children of Israel, whose ideas had become confused in their association with idolaters in Egypt, might be reminded of its terms, and understand what constitutes the true worship of Jehovah." —Signs of the Times, October 15, 1896

"The principles of the law of God did not originate at Sinai; but by a long, degrading servitude in Egypt they had become confused in the minds of all Israel. The Lord had now brought them out into this place, grand with solitude, that he might more clearly impress upon their minds the nature of his requirements by speaking his law with an audible voice. They were here to receive the most wonderful revelation ever made by God to man. The cloud which rested upon the mount, enveloping the Father and the Son and the retinue of holy angels. . ." —Signs of the Times, May 6, 1880

"It was Christ who, amid thunder and flame, had proclaimed the law upon Mount Sinai. The glory of God, like devouring fire, rested upon its summit, and the mountain quaked at the presence of the Lord." —Mount of Blessing, p. 45