I really depends on the fish, and your filtration. If you put a fair amount of live rock in there (40-50 lbs or so) and use a skimmer, you can probably get away with pushing the limits a bit more on fish than if you just stick with the Biowheel and no skimmer. Either way you go, you'd probably be fine with a couple clowns and another small fish like a firefish or goby.
The "inch/gallon" rule for fish can give you a rough guesstimate of how many fish you can stock, but is really subjective. Some fish are small, but messy eaters and messy poopers, which will obviously have more impact on water quality than the same size fish that doesn't eat or poop as much. With that said though, the "old" rule of thumb was 1 inch of adult fish size (not the size your fish is now) per 5 gallons of tank. I've seen other authors that talk about 1 inch of fish per 2 gallons of tank size. So I guess somewhere in between there is a good starting point. But again... it's not a good formula because some fish just need a bigger environment than others, regardless of size.
Coming up with a stocking list seems like it should be easy, but it takes a lot of research and time. But I found it kind of fun. Learned all about a bunch of fish that I never knew anything about. I think my original list changed about 3-4 times over the 3-4 months I was researching! And then it changed a couple more times as I started to stock my tank!
