Answer to the question:

How many Beings were involved in the plan of redemption before the foundation of the world?

Answer. Two Beings. The Father and Son.

"The plan of redemption was arranged in the councils between the Father and the Son. Then Christ pledged himself to render an account for man if he proved disloyal. He pledged himself to make an atonement which would unite every believing soul to God." —Review and Herald, May 28, 1908 par. 12

"Father and Son are pledged to fulfill the terms of the everlasting covenant. God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life." Christ was not alone in making his great sacrifice. It was the fulfilment of the covenant made between him and his Father before the foundation of the world was laid. With clasped hands they had entered into the solemn pledge that Christ would become the surety for the human race if they were overcome by Satan's sophistry." —The Youth's Instructor, June 14, 1900 par. 6, by Ellen G. White.

“Christ, the Word, the only begotten of God, was one with the eternal Father—one in nature, in character, in purpose—the only being that could enter into all the counsels and purposes of God.” —Patriarchs and Prophets p. 34