There really is no ratio in saltwater like some freshwater people use. A lot depends on what fish you want, how large they will get, swimming habits, how much waste they produce, your filtration capacity, etc.
When trying to figure out what you want , places like Liveaquaria have good suggested tank sizes and compatibility charts. Stick to those guidelines and don't go with the "I've seen people with Yellow Tangs in 30g tanks, so mine will be fine " and you should do okay. Also, be careful with LFS
advice, as they often do the "it should be fine" and leave out the "...but not for long."
When you come up with a possible list, post it here for feedback.