Grand Theft Auto Online apparently has around 500 missions for you to do.
This comes from Rockstar Games director Leslie Benzies during an interview with S3Djuegos.
I think they are about 700 missions at this time. We hope you have a 500 always available, and are renewed continuously. You can also go to the Rockstar Social Club and get off more missions. So you can connect and download any player missions to populate your world online, both from friends and from other users.