70 "Normal missions" according to CVG
but what people don't notice is that each heist has 3-5 missions of preparations.
Assuming these missions aren't classified as a "normal mission"
Than the total mission count would be
70 Normal Missions
6 Main Heists
20+ Heist Preparation missions
= 96+ Main story missions
Now for anyone who was worried would stop having a panic attack as (atleast in my contrived calculation) it is about as long as GTA IV