The relative number of mitochondria in a cell would be a general indicator of the extent of energy (ATP) producing occurring within that cell.
Mitochondria, during the oxidative phosphorylation, uses oxygen available within the cell to convert chemical energy to energy in a form usable to the host cell (ATP energy). The number of mitochondria in a cell can vary between different cell types. Cells which require more ATP will have more mitochondria (for example, liver cells can have more than 2000 mitochondria, heart muscle cells even more than 5,000 mitochondria).