Specified the number of used (in blue so it's easy to track to)

Burn damage is left undefined as far as flesh goes cause it really depends on how long something is permitted to burn. Unchecked, I supposed flames could cause some serious damage, but I tend to try to leave that up to the other PC, as they would have the control over how long up to the length of time the fireball burns. For example the blue fire burns up to 5 seconds. At 5 seconds the magic fire goes out (unless otherwise noted as having been on a flammable thing like dry grass or whatever). It can be put out before that 5 seconds, so damage to flesh can be minimized by the affected player. If it catches their shirt, it would behoove them to get the flames out as quickly as possible since a shirt isn't going to be much of a barrier between flame and skin. If it lights up armor, and they aren't fussed about scorching or the heat, they might let it fizzle out on its own.