Far Cry 5 has 8 Clutch Nixon Stunts. For the trophy / achievement "The Greatest SOB That Ever Lived" you must complete one Stunt in every region.

All of the Clutch Nixon Stunts are available from the beginning (after Prologue). They get marked on the map when you get close to them. There are 3 regions total, so you must complete 3 stunts for the trophy (one per region).
They also unlock new vehicles for your garage (seaplanes, cars, boats) and also a lot of cash and 200 Resistance Points. None of them are missable, you can do them at any point in the game. They consist of hitting checkpoints in a certain time limited. Each stunt features a different vehicle (planes, cars, wingsuit, quad).

Joseph's Region:
#1 - Spray and Pray - 0:05
#2 - Baptism of Fire - 3:15

Faith's Region:
#3 - Decente Dans La Folie - 5:17
#4 - Old Glory Holes - 7:39
#5 - Godspeed - 10:33

Jacob's Region:
#6 - The Mooseknuckle Run - 12:37
#7 - The Lord of the Wings - 15:46
#8 - Quadzilla - 17:21