Actually, the mercenary line has potentially unlimited promotions.
I believe it goes mercenary - > sword master - > magic knight - > villager - > one of four promotions, one of which is mercenary - > repeat cycle as much as you like.
I also believe cavaliers (who are simply called knights in the J2E translation) have two more promotions. I believe it goes knight - > paladin - > gold knight.
A villager can promote to either a mercenary, knight, archer or soldier (not sure I remember which), or a magician. Like I stated above, the mercenary path has unlimited promotions.