Except not many people go by that rule.
Try the aquarium calculators in the pinned topic. Because it's a longer tank, there's more surface area for gas exchange and you can get more fish.
Personally, I can put roughly 28" in my 15 gallon, because of the surface area and the larger filter. It works out well, and I've only seen a slightly higher level of ammonia once, due to being late with a tank cleaning because of a paper that was due.