12V headlights are DC current and will not work on AC nomatter what the voltage you try to push through them. Just get some halogen drop lamps if you're running them off of a generator.

If you did somehow manage to get 120 Volts DC (10 batteries in series, not too economical IMO) the formula is I=V/R I being the amperage, V being the voltage, and R being the total resistance of all the lamps together.