As a human is made up of billions of cells, so can we take them as many souls or a single soul?

Each cell in the body is a soul, so the body is made up of countless of living entities. But there is only one self of the body, and that is you. In this way we can understand what a rare occurrence it is for a soul to take birth in a human body.

The birth in a human body is a unique opportunity for the soul to realize itself, and re-develop its lost love for God.

The evolution talked about in Lord Brahma’s statement here, is an evolution of consciousness, of course, not an evolution of material bodies. The soul evolves its consciousness by transmigrating through different bodies - fish, plant, insect, reptile, bird, animal, and human - not that an amoeba gradually grows legs and learns to talk, as atheists would have us believe.

“One attains the human form of life after transmigrating through 8,400,000 species by the process of gradual evolution. That human life is spoiled for those conceited fools who do not take shelter of the lotus feet of Govinda.”

--Brahma-vaivarta Purana