Does an overweight person usually have more fat cells than an underweight person ?

The difference between obese people and thin people isn't the number of fat cells; it's the size of them. You don't make more fat cells the fatter you get; you have the same number of fat cells you had as an adolescent. The only difference is that the fat globules within each cell increase as you store more fat. By the way, muscles work the same way; you don't make more muscle cells; the muscle cells get larger.