This is cumulative so it does not have to be done in one playthrough. Just begin doing work for one of the four job brokers in either Castle Amana, Castle Minori, Takatane Village or Omiki Town. These brokers are easy to find as their names are in a pink background. Also keep in mind that in order to unlock new missions from these brokers you will have to advance in the story.
It does not matter how many times you repeat the same mission as long as you complete 50 missions. Many missions will have to be completed in order to obtain certain endings, get specific weapons and possibly earn money (If you need money) so this achievement will pop up through normal play.