Answer to the question:

How many Beings were involved when God said, "Let us make man in our image"?

Genesis 1: 26 Then God said, "Let us make man in our image, after our likeness. . ."

Answer. Two Beings. The Father and Son.

"After the earth was created, and the beasts upon it, the Father and Son carried out their purpose, which was designed before the fall of Satan, to make man in their own image. They had wrought together in the creation of the earth and every living thing upon it. And now God said to His Son, “Let us make man in our image." —The Spirit of Prophecy, Vol. 1 (1870), p. 24.

"God, in counsel with His Son, formed the plan of creating man in His own image." —Review and Herald, February 24, 1874 par. 3, by Ellen G. White.