The more mitochondria that are present in the cell, the higher capacity of energy usage.
You could devise an experiment like this:
You grow two types of human cells in a petri dish, one with high mitochondria count and one with low mitochondria count.
Add a fixed amount of glucose to both dishes and over time you'll observe a difference in glucose concentration.
This could be your evidence.
Liver cells tend to have a high mitochondria count because they have a lot of work to do in the animal body (mostly detoxing blood)