You are sitting at a level 6 so you have a bit of power levels to play with. I'd suggest something akin to 2-3 uses for each fireball or an overall 7 or 8 uses, with which you could choose what fireball you want to use at the time, depending on the situation.

Also for the general burning ability, are you thinking a major burn (going down to flesh) or more minor (e.g. skin only)? Or is this effected for the length of time it is touching the person?