To calculate the current draw of an amplifier, multiply the number of channels by the RMS watts per channel (a 2 channel amp rated at 2000 watts RMS per channel would be 4000 watts). Double it to account for amplifier inefficiency (4000 watts X 2 = 8000 watts), then divide by the average output voltage of an alternator, 13.8 volts (8000 divided by 13.8 = 579 amps). Since the average music signal requires about 1/3rd of the average power in a test tone, divide by 3 (579 amps divided by 3 = 193 amps).

This is an average approximation of current draw and can go higher. So I would gather between 194 and 289 amperes for this example.

The alternator of a:

compact car is about 35 amps
mid-sized car is about 65-75 amps
SUV is about 145 amps

Calculate 40% of the amperes of the alternator you have now. This is the average current the car uses when running. Add this number to the approximate average current draw of the amps, and you will come up with the actual size of alternator you need.

If the number you come up with is the same as what your alternator supplies, you need to take no action.