In a regulated mod, series vs. parallel doesn't really matter much. The calculations are different, but the results are the same. Let's take an example. I'll use round numbers for ease of mental math. So let's say you have two batteries, each at 4.0 volts. You're vaping at 80 watts. (Resistance doesn't matter on a regulated mod. Note it's completely absent from the calculations below.)
If the batteries are wired in series, battery voltages are added together:
4.0v + 4.0v = 8.0v
80W / 8v = 10A
Batteries wired in series "feel" the full amp draw for each battery. The amp draw is 10A per battery. If, on the other hand, the batteries are wired in parallel, the voltage does not "stack" like this, and so the voltage is just the same as that of a single battery:
80W / 4v = 20A
However, batteries wired in parallel "split" the load evenly among all batteries. So the amp draw in this case is, still, 10A per battery.