Why do you need at least two balls to play tennis?
You don't NEED two balls to play tennis, it's just helpful to have another in your pocket to not spend an extra 30 minutes chasing tennis balls.
Well, to start off, you need a tennis racket! You may also want to consider a tennis bag, water bottle, shock absorber, water bottle, visor, etc. These are the main things you will need, but you might need some other things like tennis balls, a person to play with, and if you are just starting tennis, maybe a teacher