There are a total of 16 training missions.
The first two are present when you start the game. each mission you complete then unlocks another mission on the list.
You have to complete 14 of the missions to get all 16 unlocked.
a mission is only counted complete when you have earned a bronze, silver, or gold star for the mission. Most missions are on average 4 minutes long, this achievement should take little more then 1 hour to complete
note: you should have little trouble getting at least bronze the first time you attempt a mission.