How do you determine the number of cells in a battery?

It depends on the chemistry of the battery. A common alkaline or zinc chloride non-rechargeable battery will be about 1.5v per cell. a NiMh or NiCd rechargeable battery is usually quoted at 1.2v per cell, Lithium Ion cells are generally quoted at 3.7 volts and lead acid at 2v per cell. Divide the pack voltage by the individual voltage of each cell and you will find out how many cells are in the pack.