There are several approaches to your solution, but they all boil down to estimating the expected maximum current draw then adding a reasonable margin for charging the battery(ies). A grass roots approach is to add up the current draw for every electrical component you expect to operate at any given time (ie, lights, w/wipers, heater-A/C blower, amplifiers, etc... don't forget the winch and trailer lights, if you have them - you get the idea). If you know the wattage of a device but not the current draw, you can calculate the current draw by dividing the wattage by the system voltage. Add all the current draws together, then add an "arbitrary" 40-amp margin per battery for charging.
So, let's say you have two 100 watt headlamps (100 w/12.6 v = ~8 amps, X2 = 16 amps) + blower motor (~20 amps) + w/wipers (~20 amps) + tail/running lights (~1 amp/bulb x 10? = 10 amps) + amp (~250 watts or ~20 amps - although you will rarely run an amp over 70 watts while driving if you ever wish to hear again) = 86 amps. Add ~40 amps per battery for charging: 86 + 40 = 126 amps. So, you can "estimate" the minimum alternator at 130 amps. Another method is to insert an ammeter between the battery and either cable (observing proper polarity) and turn on every electrical device you would run at one time, and measure the load directly. Then add the margin as before, per battery, for charging the battery(ies). This is not a strict formula, but it is workable and allows some flexibility.
A major issue, however, (often overlooked) is wire size versus current flow through the wire. The more current flowing through a conductor, the greater the effective resistance of that conductor and the more voltage that is used "across the conductor" forcing the current to flow through the conductor. If you increase the current demand of a system, you must also increase the wire size. There is a specific formula, but charts are readily available for selecting the wire size based on the current load through the wire with respect to the length of the wire run. It sounds complicated, but really isn't. With a 130-amp alternator, I generally run a 4-gauge copper charge cable to the battery (assuming a run of no more than three feet); all cable ends are crimped and then soldered. Incidentally, don't forget that the ground is half of your circuit.
So, there you have some food for thought....
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