Kinda sounds like a 29 gallon tank to me if the measurements are correct.
Regarding the amount of fish you can put in... it will vary greatly depending on how many gallons your tank actually is. If it's 29g, you're not going to be able to put all those fish in there, without something really really bad happening. If it's a 50g tank, then you *might* be able to squeak that many fish in there - as long as you don't put in the damsels.
To determine tank capacity, measure length, width, and depth (all in inches) and multiply them all together. Divide that number by 231, and that's how many gallons (roughly) you've got.
One note... your cleaning crew (snails, crabs, shrimp) don't really add to the bioload of the tank, so the amount of cleaning crew doesn't effect how many fish you can add.
I'd suggest getting a couple books to supplement your online research. The first is Michael Paletta's "The New Marine Aquarium", and the second is Robert Fenner's "The Conscientious Marine Aquarist". The first is a pretty easy, quick read that is a good overview of the hobby. The second is a good, in depth book that covers things in a little more detail. A lot of things probably won't make sense when you read them the first time, but as you get into the hobby more things will make sense.
And welcome to Aquarium Advice!
Oh... and it's a "Royal Gramma".