So, how many fish to begin with? It depends a lot on a few factors: How big is your tank and how healthy is your biological filter? The "One Inch of Fish per 5 Gallons of Water" Rule of Thumb generally applies to seasoned tanks with healthy filter.
Depending on the size of the tank, start with one or two small fish in a 55 gallon tank to start with. This will supply the food (ammonia) needed to start the bacteria multiplying and populating your filter. Consider one or two of the Terrific Beginner Fish to help you get started. Look at the Saltwater Aquarium Fish Compatibility Chart to see how your fish choices are going to get along in your tank.
If you are going to use a good Nitrifying Bacteria Tank Starter (highly recommended) to help with the cycling process, one or two small fish should present no problems in your new tank. Follow the directions on the package as to how much to use and when to use it when adding nitrifying bacteria. It is recommended that, if you are using a protein skimmer, you turn it off during the initial cycling period. This will help the bacteria spread in your biological filter.
When you add each new fish or invertebrate, make sure that you take your time acclimating the new arrival. Chances are that the pH of the tank water where you are buying your livestock is a lot different than that of your newly set up tank.
If you have tank lighting which will support corals, you can also add a coral or two to your tank at this time. The corals listed in 15 Easy Corals for Your Reef Tank is a good place to start for corals which are fairly easy to maintain and are more forgiving of less than perfect water quality and somewhat lower lighting.
Next we are going to Add More Fish & Corals