Why Tennis Balls are Packed in Pressurized Cans?

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.

Why is there a number on tennis ball ?

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.