Instead of many small ones, why does a cell not have a single giant mitochondrion? How does this relate to the concept of surface area?

A cell has many small mitochondria instead of a single giant one because many small mitochondria together provide a much larger total surface area than a single large mitochondrion of the same volume.

This relates to the concept of surface area as follows: as an object becomes larger, its volume increases faster than its surface area, so the surface-area-to-volume ratio decreases. Mitochondria carry out cellular respiration on their inner membrane, which is folded into cristae to increase the surface area for chemical reactions and energy production. Many small mitochondria maximise the surface area available for these reactions, allowing more efficient release of energy (ATP). Additionally, several small mitochondria can be distributed throughout the cell to supply energy wherever it is needed.