electrical system upgrade

The equation is good ol' Ohms Law.  Power is the result of voltage X amperage.  Voltage is the car system, and you can figure 12 volts for that.  If you were in with some competitors who beef up voltage to a higher number for comps, you would already have an inside track to what size alternator you'd need to get.  Batteries parallel-linked together still provide the same voltage, so there's no need to consider them in what you would need in an alternator.  You also have to consider what the car needs in power and then provide above and beyond for the sound system.

Amplifiers waste some of the power they receive, to boot.  Figure at least 30 to 40% waste in heat...so if you want 5000 watts of power you need to be able to feed the system enough current to equal about 7000 watts when multiplied times voltage.  7000 / 12 volts = 583 amps.

Realistically, what is it that you want to achieve?  Or is your question rhetorical only?

i am asking, because i would like to end up with a system total of about 4k watts. When you talk about waste in heat, is that related to the amplifier efficeny at all? my amps would be RF 2000BD which should have an efficency of about 80% how would i figure it out then????

4000/.80 = 5000watts. then 5000/12 = 416 amps, would that be correct at all?????

I wanna run (4) 15" L7's with (2) RF 2001BD's