That's a very good question, but it's also a hard one to answer. A lot depends on your personal goals and personal abilities. Because we are most interested in the fitness, health, and wellness of teens, we encourage participation in a variety of activities from the Physical Activity Pyramid, not just one sport. Each of the five types of activities from the first three levels of the physical activity pyramid has different benefits, so performing a variety of activities will provide you with a variety of benefits in fitness, health, and wellness. Also, people who learn a variety of activities and perform them regularly as teens are more likely to be active and fit later in life. So if your goal is good fitness, health, and wellness both for now and later in life, we suggest learning many activities.
It is true, however, that many successful athletes specialize in one sport. They think that by specializing they can spend more time practicing to become really skilled at one thing rather than divide practice time among several sports. So it can be argued that if your goal is to be really good at one sport, specialization can work. The problem with that approach is that many young adults do not continue to play a sport after they get out of school. So people who learn only one sport may be better at it when they are young, but they may not be active later in life because they can do only one sport well.