Mitochondria is a powerhouse of cells which are generally involved in the production of energy currency which is ATP.
All the cells require energy to live and therefore mitochondria are found in all the living cells whether plants or animals. Since mitochondria are found in both plants and animals.
The number of mitochondria in a cell is also dependent on how much energy that cell normally requires.
For example, muscle cells that are involved in locomotion or lifting weights will normally require more energy and more ATP than a skin cell.