All children in the districts from ages 12-18 have their names entered into the Reaping Ball. The number of names entering the Reaping Ball depends on several factors. First, older children tend to have more slips than younger children, as the votes are accumulative (meaning, if you placed your name once in the ball when you are 12, you enter it twice when you are 13, three times when you are 14, and so on...). Second, it depends on whether you are of the wealthier class or of the poorer class. When placing your name in the Reaping Ball, you have the option to apply for a tesserae, a year's worth of grain and oil for one person. (This helped Katniss survive). However, when you apply for a tesserae, you also enter an additional slip into the ball, increasing your odds of being called.
For the 74th Hunger Games reaping, Katniss's name was in the ball 20 times Gale's name was in the ball 42 times