The basic math behind it is Ohm's law:
Volts x amps = watts
So 12v*200amps = 2400 watts
or 6v*200amps = 1200 watts
So the overall energy for a 6v is 1/2 that of a 12 volt. Two 6v would get you to 2400 watts, same as the example 12v.
BUT - usually pairs of cheaper "golf cart" batteries have higher amps than many 12 volt batteries. And they can be discharged deeper.
So it becomes a game of balancing cost, capacity, available space, and weight.
In my case, two 6v gave me more capacity than what I could fit in the available space with one big, or two small, 12 volts. And they were a bit cheaper.
Some people go with 2 or 4 6v, some are happy with 1 or 2 12v. Depends on your priorities for the 4 selection criteria listed above.
2012 Ford F-150 Eco, Max/Max
Jayco 26.5 RLS, Reese Sidewinder