How do you access Mission 46?
According to this link, you are needing to have:
Complete all of the main story missions (excluding the repeat missions) before and after this mission.
You must also complete all of the important Side Ops indicated in yellow.
Listen to all of the yellow cassette tapes
You will need a fully upgraded Mother Base and all of its platforms.
If I remember correctly, there are about 49 total missions but Mission 46 marks the end of the story, Truth.
Due to comments, I will probably say a nearly fully upgraded Mother Base instead of a fully upgraded one. Maybe this link could help you unlock mission 46: http://www.gamefaqs.com/boards/718564-metal-gear-solid-v-the-phantom-pain/72466817
The text in it:
To unlock Mission 46 "Truth: The Man Who Sold the World ", Phantom Pain's true final mission you need to meet certain requirements.
- Complete missions 1-31
- Complete postgame Story Missions
Mission 32 To Know Too Much
Mission 38 Extraordinary
Mission 41 Proxy War Without End
Mission 43 Shining Lights, Even In Death
- Complete postgame Side Ops Story Missions
Side Op - Extract the AI Pod
Side Op - Secure the Remains of the Man on Fire
Side Op - Search for the Escaped Children 1
Side Op - Search for the Escaped Children 2
Side Op - Search for the Escaped Children 3
Side Op - Search for the Escaped Children 4
Side Op - Search for the Escaped Children 5
- Listen to all story audio/cassette recordings (they are yellow in colour)
- Upgrade entire base, and platform to level 3 or higher.