The whole PV situation revolves around this 2 equations
V=IR
P=IV
Where V=Voltage(V), I=Current(Ampere), R=resistance(ohm), P=power(Watts)
Typically current draw is dependent on resistance of coil and voltage where current is the factor in consideration for safety(battery safety amp limit).
To put it simply current is the force needed to push a voltage across the resistance.
you can place series
batteries with no limitations for diy but typically for dual 18650
mods the top end will be 8.4V. The ideal "max" is the current that your battery can push, pushing too high will cause overheating of battery.
The resultant use of power is confined by type of wire, heat flux and heat capacity which is of another topic altogether that affects vaporising the liquid itself.