How many main story missions are in Metal Gear Solid 5: TPP?

Solution 1:

41 unique story missions in total (Mission list in spoiler below) and 157 Side Ops.

Mission 1 Phantom Limbs
Mission 2 Flashback Diamond Dogs
Mission 3 A Heros Way
Mission 4 C2W
Mission 5 Over the Fence
Mission 6 Where Do the Bees Sleep
Mission 7 Red Brass
Mission 8 Occupation Forces
Mission 9 Backup Back Down
Mission 10 Angel With Broken Wings
Mission 11 Cloaked in Silence
Mission 12 Hellbound
Mission 13 Pitch Dark
Mission 14 Lingua Franca
Mission 15 Footprints of Phantoms
Mission 16 Traitors Caravan
Mission 17 Rescue the Intel Agents
Mission 18 Blood Runs Deep
Mission 19 On the Trail
Mission 20 Voices
Mission 21 The War Economy
Mission 22 Retake the Platform
Mission 23 The White Mamba
Mission 24 Close Contact
Mission 25 Aim True Ye Vengeful
Mission 26 Hunting Down
Mission 27 Root Cause
Mission 28 Code Talker
Mission 29 Metallic Archaea
Mission 30 Skull Face
Mission 31 Sahelanthropus
Mission 32 To Know Too Much
Mission 38 Extraordinary
Mission 41 Proxy War Without End
Mission 43 Shining Lights, Even In Death
Mission 45 A Quiet Exit
Mission 46 Truth: The Man Who Sold the World
Mission 47 The War Economy
Mission 48 Code Talker
Mission 49 Occupation Forces
Mission 50 ST-84 Sahelanthropus Boss Fight
Mission 51 Kingdom of Flies