I was curious about this myself earlier this afternoon and after some googling this is what I learned. In the Atlantic they have 6 lists of 21 names that they rotate every six years. If a storm is particularly historic, they retire that name and replace it with a new one six years later. If there is more than 21 storms in a given year, then they move to the greek alphabet (ie. #22 = Alpha, #23 = Beta, etc.). The most on record in the Atlantic is 1933 with 21 (I think 1995 had 19). The pacific is very similar except that several years ago (maybe 15), they actually got to W and decided real quick to add names for X, Y, and Z... they used all 24 that year. After this they kept the first 21 names alternating every 6 years and added X, Y, and Z names that alternate every other year. If they go past 24, then they will revert to the greek alphabet as well. I'm sorry I can't remember the URL for this, but if you search on google for "21 storms" I think you'll find something eventually.