The inch "rule" doesn't apply to small 1-2inch fish, and ember tetras only grow about 1- 2cm! Tetras are tiny fish that use little oxygen and produce far less waste than a larger fish. I've seen a hundred or more tetras (neons) in a tank half that size and they all seemed fine to me (there were no plants or decorations though). But definitely don't add them all at once, like leighton said add about 20-30 at first and slowly build up the numbers.
I think the main factors are the type of tetra(s), how many plants (more plants means less space), how many decorations you plan on having and what shape the tank is (I believe longer tanks rather than tall ones are better for tetras because tetras tend to hang about in the middle of the tank, not at the top or bottom). Currently i'm trying to set up a 160 litre tank and feel I could comfortably house 50 small fish (i'm thinking neons, possibly a shoal of 10 or more glowlights, a dwarf gourami and some khuli loaches and probably some cherry shrimp because they take up hardly any space at all), (the fish I have planned also take up different space in the water column). For me, in a 260 litre tank I would imagine 80+. If I were to heavily plant or decorate my tank these figures would probably be lower.
My tank came with a fancy goldfish who's since been rehomed and he was massive, about 5 inches either way and he never stopped going to the toilet. His waste was bigger than most tetras, I wouldn't try and get more than 3 - 5 of those in my tank! (I think that's where the inch rule comes into effect) Tetra waste on the other hand is so small and hardly noticeable.
You'll know how much your tank can take as you stock it up gradually, I suspect when you put the first bunch of tetras in you will see just how small a space they take up.