Lets say I have an EC
Lord charge a unit of necrons.
"Until the end of that
phase, if an attack made by a model in that unit
destroys an enemy model, the attacking model can
make one additional attack against the same unit
using the same weapon."
if each attack gets a kill and I have 4 attacks that suceeded, then I have 4 more attacks. I don't see anything that limits extra attacks from creating more extra attacks, so as long as I kill a model each time I could wipe an entire unit of 20 necron warriors? Or is this basically written that I'm supposed to know that I get 1 bonus swing and I'm done?
You can get infinite attacks from that, yes. Provided you keep killing and killing and killing.
For instance, if you're attacking a squad of Conscripts (T3, 5+) with a Lightning Claw on a unit that has Veterans of the Long War, Prescience, and RR1s to-hit from a Lord or something, you'd expect to get at least a dozen swings per attack made, since each swing has a 94.5% chance of killing a guy.