There are a total of 50 main missions, all of which are required for this achievement, these include all of the repeated extreme, subsistence and total stealth missions.
Two of these missions require some work to unlock.
Mission 45 - A Quiet Exit
To unlock this mission you will need to;
- Max your bond with Quiet (playing main story missions)
- Not use the butterfly on you Motherbase emblem
- If it hasn't unlocked and you have finished all other missions, replay some and also play the extreme, subsistence and total stealth missions. Guides can be found on YT if you are having a difficult time with them.
- You need to trigger all quiet cut scenes, if your bond is max all you need to do is complete a side op and return to base a to trigger them, do this enough and they will all trigger.
Mission 46 - TRUTH: The Man Who Sold The World
This one takes a little more effort;
- If you are early into the game make sure you you are constantly leveling up your Motherbase.
- I had at least 2 facilities on every platform, I also had at least 2 platforms with 3 facilities. Keep in mind this may not be required you might unlock this earlier.
- Finish all other main missions (not extreme, subsistence and total stealth) you dont need to finish A Quiet Exit either.
- Finish all important side ops, these are the yellow side ops.
A couple of things worth noting, Quiet it the best buddy in the game, I highly recommend using her as soon as you meet her as she wont come back after mission 45, using the butterfly in your emblem will make her stay with you until you change it and return to mother base.
I had a friend missing mission 46 without realizing until I pointed it out. If you are familiar with Metal Gear Solid: Peace Walker, you will understand how and why this mission is hidden like this. Peace Walker is a lot more of a grind than The Phantom Pain.
After you have met all requirements for both the missions, you will get a call for A Quiet Exit from Kaz. Truth: The Man Who Sold The World will just appear after returning to the ACC.
Good luck and please comment with any problems or with anything I got wrong.