+2 is the most common and basic bonus from stunts. The "Mortal Stunts" chapter actually goes through the various bonuses that's within the scope of a stunt. They are mostly:
2 shifts on non-attack stuff (can be a straight +2, or an extra Mild Consequence, or 2 time increments faster, etc)
+2 to the effect of a successful attack
+1 to perform an attack under a specific circumstance
Or a stronger effect at the cost of a Fate Point.
There might be more options, check the beginning of the chapter on Stunts.