There are a total of 6 multiplayer maps, and 6 N7 missions.
SP: The N7 missions are not story related and are optional quests found within the campaign. You must actually do them all before you finish the game or you will have to resort to multiplayer in order to gain the achievement.
MP: You just have to finish all of the waves for each map. This can be done on the Bronze difficulty to make things easier. Best done with a full team and with a few levels under your belt to have upgraded weapons and powers.