Cadence is the number of revolutions the crank is turning per minute. It is equivalent to a car tachometer. Simply put, it is the amount of revolution that a cyclist is able to turn his crank for a sustained period of time. While there are many articles written on what is a good or bad cadence for riding and how to maintain a high cadence, I personally believe that there is no special number that 1 must reach. In the Lance Armstrong days,it was emphasized and drilled to beginners that one must keep a cadence of above 90. Lance Armstrong was famous for keeping a very high cadence (>100) for his tour periods.
Yet, personally I tried keeping a high cadence but I found it very exhausting, my lungs were crying out and it made me exhausted pretty fast.So I switched to lower cadence(70-85rpm) and found it easier to manage. Going faster was easier on the lungs and though not as easy as on the legs. But my legs are somehow able to cope with the pain more than I could cope with the pains from my lungs!
Different people have different anaerobic and aerobic capacities, while I do not have any advice for keeping a specific number, to improve however, requires one to be able to keep a good tempo. This can be done by having cleats as well as having a good cycling position.While I prefer slightly slower cadence for cruising on the flats,I like having higher cadence on the hills as lower cadence is harder for me to maintain while climbing up.