In the Dragon Ball: Bouken Special, the Dragon Ball manga author, Hirohiko araki, explains that there are seven Dragon Balls because there are eight balls in Hakken-Den, a famous Edo-era Japanese novel, and that he would have hated for them to be exactly the same number, so he had the Dragon Balls be seven.
He spends his childhood far from civilization until he meets a teen girl named Bulma, who encourages him to join her quest in exploring the world in search of the seven orbs known as the Dragon Balls, which summon a wish-granting dragon when gathered.