Balls are pressurized above atmospheric pressure (more inside pressure than outside) to give perfect bounce and rubber is not perfect to keep the pressure at the manufactured level. So cans are pressurized and sealed to maintain pressure at the factory/best level.
Dunlop, WIlson, Slazanger, Penn, Babolat, Viking Sport
Balls are numbered to identify balls easily. Whether playing at common courts so ball won’t mix up with others or to remember which ball you were playing/practicing after putting them back in can.
It’s answer is not simple, please read guide once.
Normally a tennis ball contains 3 balls and costs $2-3 USD. Some professional balls cost more than this.