Final answer:
Different cells have different numbers of mitochondria based on their energy requirements. Muscle cells need a lot of energy for contraction, so they have a high number of mitochondria. Red blood cells, on the other hand, do not have any mitochondria.
Step-by-step explanation:
The number of mitochondria in a cell is determined by its energy requirements. Some cells, like muscle cells, require a lot of energy for contraction and therefore have a high number of mitochondria. Other cells, like red blood cells, have no mitochondria because they do not need energy to perform their function. Additionally, the number of mitochondria can change in response to the cell's needs. For example, if a cell needs to produce more energy, it can increase the number of mitochondria.
Learn more about Mitochondria in cells