The rules actually more about one inch per gallon but many people do not follow this it tends to depend on the type of fish your keeping, in general the rule should works, in terms of a filter for a ten gallon can handle the bioload of one inch of fish per gallon, but different fish create varying degrees of waste and ocupy different spaces in the aquarium, a well planned aquarium with a variety of bottom dwellers, middle swimming fish and, upper area fish, with maybe a little extra filtration or water changes can hold alot more fish than one inch per gallon,
also if your fish isn't hardy like say a discus (which is too large for your tank) should be understocked as they require very good water, or certain agrisive species like african mbuna, should be overstocked to reduce agresion
So what type of fish you want?