Well technically you could have almost 60 in that talk (5 inches of tetra per gallon). I was reading Aquarium Fish Magazine and that's what was written last year. Although the writer said not to do it unless you know what you are doing, he does it himself.
Reading the new magazine put out by Hagen (I think it is called nutrafin news) it said you can put 2 and 1/2 inches of tetra fish per gallon with no problems. -that was in the very first issue ever.
I tend to agree with them, as long as you have adequate filtration.
Just go with 7 tetras or so