Playing from the ' Prologue' all the way to one of the three possible endings is required for 100% completion. You can access up to 74 missions in a single playthrough of GTA V, depending on your choices.
This includes all the Heists branching approaches, the optional Lester's Assassination Missions, and a walkthrough for all the missions in the single player campaign. Here you find the full list of all the GTA 5 Missions, along with the complete story mission guide for Grand Theft Auto V.