Final answer:
The number of mitochondria in a cell is increased through the process of fission, where preexisting mitochondria replicate, supporting the endosymbiotic theory.
Step-by-step explanation:
The number of mitochondria is increased through the fission of preexisting mitochondria. This self-replication process is similar to binary fission witnessed in prokaryotes. Mitochondria only arise from preexisting ones and are not formed from scratch by the eukaryotic cell. They may also fuse together, and they are distributed within the cell during division, which indicates their former independent lifestyle. This method of replication and distribution, along with mitochondrial morphology and possession of their own DNA, supports the endosymbiotic theory that mitochondria were once free-living prokaryotes.