e.g. will a lymphocyte in one person have roughly the same number of mitochondria as a lymphocyte in another person.
".... The number and shape of mitochondria vary dramatically in different cell types and can change in the same cell type under different physiological conditions, ranging from multiple spherical or cylindrically shaped organelles to a single organelle with a branched structure (a reticulum). The arrangement is controlled by the relative rates of mitochondrial division and fusion, which are regulated by dedicated GTPases that reside on mitochondrial membranes' In addition, the total organelle mass per cell can be regulated according to need. For example, a large increase in mitochondria (as much as 5-10-fold) occurs when a resting skeletal muscle is repeatedly stimulated to contract for a prolonged period."
Alberts, Bruce; Alexander Johnson; Julian Lewis; Martin Raff; Keith Roberts; Peter Walter, Molecular Biology of the Cell, 5th Edition.
As the Paragraph above explains, it can be assumed that due to different metabolism in different people, a specific number of mitochondria in the same cell types for different people may not be assumed.
No, because there are 3 different types of muscle fiber, each containing different numbers of mitochondria, each correlating with a different requirement for oxygen and glycogen.
You also have different muscle types throughout your body - cardiac muscle, smooth muscle, skeletal muscle.
Different people of the same group (e.g. region) will have minor genetic differences which will result in different proportions of muscles and muscle type distribution in their bodies.
Different people across groups (e.g. different countries) will also have different environments to deal with, and different strains on their muscles, which result in muscles developing differently over time. People in Kenya run for miles to get water, and require high-endurance, while people in California spend a lot of time in the gym doing weights, which require less endurance and more power.
I would expect that this is true for other cell types as well, as different environments and differences in genetics will result in different requirements for cellular metabolism, all of which will dictate the number of mitochondria needed.