An increase in the number of cells in a multicellular organism occurs by?

There are two types of reproduction, or rather cell division.
Type 1 is mitosis. During mitosis, cell ploidy is preserved, i.e. a set of 2n remains in a new cell. From one 2n cell, 2 new cells are formed by mitosis. Mitosis forms somatic cells in all organisms and gametes (sex cells) in plants.
Type 2 is meiosis. With meiosis, the ploidy of the cell decreases and the 2n cell turns into an h cell. During meiosis, 4 new cells are formed. Meiosis produces gametes in animals and plant spores.