I have an AT-RT equipped with the flamethrower. I've moved it up and there are two stormtroopers of a five man unit within Range 1 of the AT-RT. When I fire the flamethrower do I fire it at the whole unit or only the models within range 1? Do I roll four dice or ten dice? And if I get to roll ten dice are all the models in the unit viable casualties?

TIA

It doesn't matter how many or what mini is in range as soon as one mini of a unit is in range the whole unit is in range and can die from an attack. EXCEPT there are minis in that unit that your ATRT can't see (no line of sight). They can't die and give no attack dice for your flamethrower.

So given that the ATRT has LOS on every stormtrooper you throw 10 dice and everyone can die.