Well that'll only be being drawn from the decoupling capacitors near each driver chip, so it'll be spread evenly across the board. Realistically you're only ever gonna be switching 6 FETs on at one time, and if you calculate the impedance of the 6 FETs at 50kHz it's only gonna be drawing an average of about 200mA from the 12V gate drive supply, assuming a 10nF gate capacitance.