If you try to follow a new person and you see the error, “You can’t follow any more people.”, you are already following 7,500 people. That is the maximum number of accounts you can follow on Instagram.
To follow a new account you will need to unfollow some of your current friends on the service. This is to combat spam on the service. If you see people following more than 7,500 people they were doing it before the new rules took effect.