Dirt Rider, if you go down the battery charging rabbit hole you'll bump into formulas. There's probably one for what you are asking but it could be a long one, dependent on many factors.
From what little I know I'd say you'll want a system that can recharge the battery bank as much as possible, as quickly as possible. I'd look into the charge current recommendations and maximums given by the batt mfr. These may vary by make and will certainly by type.
If we use a traditional bulk charge rate of .2C (20% of the batt AH) and you had 4 x 100 AH batteries that would suggest an 80A current avail for charging as a guideline (plus some to spare for other needs while driving). Whether your oem alt can output and sustain that is another question. Transmission of that current is another requirement, albeit easier. On the surface of it tho, you might be OK.
If you choose a battery bank that can be charged at a higher C rate then a higher output alt could be useful depending on how depleted the bank is and maybe a few other factors I'm forgetting. But if you drive a lot, might be less of a benefit.
If you do choose to upgrade and can do so by adding a second alt that gives you redundancy - that would be a good thing.