why do some cells have more mitochondria give an example

Careful Angel - only eukaryotes have mitochondria! According to the endosymbiont theory, at some time in the distant past a eukaryotic cell and a prokaryotic cell formed a mutually beneficial relationship whereby the prokaryote took up residence within the eukaryote. In return for the metabolic support and protection offered by the eukaryotic environment, the prokaryote provided efficient oxidative metabolism.

But to address the original question, why do some cells have more mitochondria than others, the simple answer is that their mitochondrial count is in direct proportion to the metabolic / energy requirements of the cell, since mitochondria are essentially microscopic cellular power stations. Metabolically inert cells, like the shwann cells that form a myeline sheath around a nerve, contain very few mitochondria.

In contrast, highly metabolically active cells, live hepatocytes (liver cells) or myocytes (muscles cells) contain large numbers of mitochondria. As a rule of thumb, the greater the energy demands of a cell, the greater the number of mitochondria it will contain. "I never forget a face, but in your case I'll make an exception"
  Mitochondria are the cell structures (organelles) which are responsible for releasing energy from glucose (sugar) in a cell.

The number of mitochondria in a cell will de pend on how much energy that cell needs.   Skin cells basically form a protective layer on the body.

They are produced by a layer of cells beneath the skin which divide to produce new cells. The cells above get pushed gradually to the surface. As this happens they become waterproofed by filling with a substance called keratin, and gradually die. So the surface layers of your skin (the bits you can see and touch) are in fact dead cells! As they are dead, they do not need any energy.   Cardiac cells are muscle cells found in the heart.

Their function is to repeatedly contract, providing the heart with the ability to pump blood around the body. So they will need a continuous supply of energy.   If you have read this far, you should now be able to work out which kind of cell has the most mitochondria.