Because all of the cells in a multi-cellular organism come from a single cell, all of its cells will have the same number of

Chromosomes

Explanation:

Multicellular organism are organisms that have more than one cells in their body. However, from the very beginning of every living organism including multicellular organisms, only ONE cell is required. In the case of multicellular organisms, this one cell undergoes division by mitosis to form other cells.

Since the cells divide by mitosis i.e 1 forms 2, 2 forms 4 etc.,. each of the cells are genetically identical to one another. Hence, this means that all the cells will contain the same number of chromosomes in their genome. For example, a dog as a multicellular organism has cells that emanate from one cell. If that one cell contain 39 chromosomes, all cells in the dog will also contain 39 chromosomes.