The general rule of thumb is to buy two speakers for every inch of tower height. For example, if your tower is 6 inches tall, you will need to buy at least 12-16 JL Audio marine speakers or 8 Rockford Fosgate marine speakers or Kicker Marine KIC-86583 10W7 marine speakers. There are a few other factors that come into play such as boat size and port configuration, but this gives a good overview.