To determine the number of bacteria in the lab could be done in several methods, but the most common used are: spectrophotometric analysis and viable plate.
The spectrophotometric analysis is not a proper determination of the bacterial numbers but measuring the amount of cells, dead and living ones, could estimate the growth of bacterial population by the fact that the more increase the number of bacteria the less transmitted light will be.
The O.D.(optical density) method has several advantages, one of them being that this method is involving less technique and time.
But there are some disadvantages as the impossibility of separating between live and dead cells and the measurement at limit concentration are conflicting.