So Fire I would be a sort of cantrip, light a candle, sustain a small flame for a short period of time without fuel?
Fire II would be used for utility still. A blacksmith's apprentice would know up to Fire II most of the time, a sole fire mage in a town would know up to this to keep up with the demand of the town.
Fire III would be what the army's spellcasters would use for artillery. of course, some foot soliders would know Fire I for utility.
Fire IV would be magic commanders and high ranking officers in the military. Upper class mages could know this spell if enough time was devoted
Fire V would be magic that a king or a high ranking ruler would have, or at the very least somebody in his court.
Fire X would be for a dragon or something similar.
Fire XX would be from a godly revelation or something that grants god's powers. It would take some serious willpower to not die from casting this.
I'm still not feeling it with the whole number idea still. I want some more SPICE.