Muscle cells contain numerous to serve their high demand for atp.

Muscle cells, also known as myocytes, contain numerous amounts of mitochondria inside of their cytoplasm. Mitochondria are organelles that are popularly known as the powerhouse of the cell because they function to process different biochemicals (such as glucose, oxygen, etc) and produce essentially Adenosine Triphosphate (ATP). Kreb's Cycle in particular talks about energy production, and the mitochondria is one of the main units where this cycle happens. You can think of ATP as the currency of the body in terms of energy. The more ATP one has, the more energy there is available for use by the different systems of the body. Muscles in particular have high demand for ATP.