How often do they change tennis balls during a match?

Answer 1

That depends on the federation that the event is played under. In tournaments played under the International Tennis Federation rules, balls are changed after the first 9 games, then after the next 11 games, then after the next 9 games, then after the next 11 games, and so on. In ATP and WTA tournaments, balls are changed after the first 7 games and then every 9 games after that.

Answer 2

Every 6 and 9 games

Answer 3

Every nine games.