Yeah, i know about Priests, but I have no coupons, so...
Even if you think 15 is too much (actually never used them all before), but 1 is not enough. Mages, basically, can lvlup with NG weapon till 40 lvl, but melee classes will have problem like scavengers, that will spoil recources from lowlvl mobs.
15 coupons is a reward for the class changing quest.
For example: this is popular russian la2 wiki (sorry, it's in russian) you can click Interlude tab and for every class quest it have 15 coupons as one of rewards