Gangstar: Miami Vindication is advertised as to having over 75 missions.
There are five seperate chapters in the game and approximately 15 races.
There are side missions that you can do:
This is your basic taxi driver mission. Find a taxi, hop in and start taking fares. You will need to drive fast and DONT wreck your car or get the cops after you as it will decrease your speed by a great deal. Get 10 fares in a row and get the 'Taxi Driver!' achievement.
For this mission you'll need the food delivery car which is basically a modified W4C with a burger on it. Hop in and deliver the foods. 10 foods in a row = 'Fast Fast Food' achievement.
This mission requires an ambulance. You can usually find one near a hospital, so get in and start saving lives. DONT run over the people you're trying to save and, once again, DONT wreck your car unless you want to fail the mission. 10 people saved in a row = 'Sick Ride!' achievement.