Well, the thing is, it's all about the wattage -- the total power output, and that is based on both the voltage you're set at and the resistance of the atty/carto you have attached. The ProVari can push a maximum of 2.5 amps, so keeping that in mind you can use this
Ohm's Law calculator to plug in the voltage and resistance to see what kind of wattage it's putting out and the kind of amperage it's pulling. Going for the maximum wattage output isn't necessary -- most people are most comfortable with some combination that puts out somewhere around 8-10W, some people like it a little warmer. Best case scenario, the ProVari could drive a 2.4A atty/carto to 15W output -- but keep in mind that's pretty extreme and probably something you'll only ever do in a dare. And even then there's no guarantee your atty/cart won't pop at that level.
Generally, you're quite safe with any atty/carto at or above 2.4 ohms (max 15W output on the ProVari) up to about 3.5 ohms (max 10W output). Any higher resistance than that on a ProVari and you're starting to get into "cool vape" territory. Any lower and you're reducing the max voltage you can use while increasing the amperage being drawn per volt, which is kind of pointless.