Which process occurs repeatedly after zygote formation to increase the number of cells

Mitosis

Explanation:

A zygote is formed after the fertilization of the egg by sperm. The zygote then undergoes rapid cell division by mitosis to form a morula, blastula, gastrula and then an embryo. In mitosis, the cell replicates its genetic material and organelles and then splits into two identical cells. Mitosis is required for growth and development.