The modern World Series can last a maximum of seven games because it is played as a best-of-seven series. The first team to win four games becomes champion, so a decisive seventh game is needed only when the teams are tied 3–3.
Major League Baseball has used the best-of-seven World Series format regularly since 1922, with the American League champion facing the National League champion. In a seven-game series, home-field advantage alternates under the modern 2–3–2 arrangement: the team with home-field advantage hosts Games 1, 2, 6, and 7, while the opponent hosts Games 3, 4, and 5.
Historically, the wording can cause confusion. Several early World Series were best-of-nine, including the 1919 series, and the 1907 World Series required an eighth game after an earlier game ended in a tie. Those unusual historical cases do not describe the modern format. For a current standard World Series, seven games is the maximum.