I've learned to start/stock slow and just by room available. My "rule" is that every fish should have ample "territory" in a tank.
Unfortunately, my 29 gallon is a bit overstocked at the moment. I took in a few extra fish due to the poor fish keeping of a relative. I'm getting a tank set-up and ready for the extra fish but until that tank is cycled, it will be a bit cramped.
Some hobbyists are stickers for the one inch per one gallon rule. Some even think it should be as much as 3 gallons per one inch. Me personally? I think it just depends on the type of fish and how your tank environment is. Even with my current 29 gallon situation, my fish don't seem overcrowded. Perhaps I just have a lucky mix of fish who's personalities and size have accommodated the numbers.
My rule is, if it looks and feels overcrowded, then it is. Just be mindful. If you think "well, maybe I can't fit just one more in", then don't do it. If you have to think about it, then don't add any more.