The number of sessions has little meaning. Asession can be 20min. or 4 hrs. The courses that require more of your time YMCA, NAUI will probably teach you more simply by virture of time spent in class and in water. Some agencies, PADI, SSI don't take as much of your time before your c card is issued. Learning from NAUI or YMCA gives you a lot better opportunity to earn the card you will be issued.