What is storage capacity of a TimeMachine ?

Time Machine is free backup software from Apple. There are no models.

Apple used to (but no longer does) sell Time Capsule which was hardware that could be used with Time Machine,

The capacity of Time Machine backups depends on. the size of the external drive that you use with it. See: Back up your Mac with Time Machine - Apple Support

and

Backup disks you can use with Time Machine - Apple Support

You select your own drive.

That drive does not need to be Fast, but large is better.

Recommended size is at least 2.5 times the size of what needs to be saved, for long-term trouble free operation.

... but if you have a smaller drive available now, start now. You can add an additional drive later, and then every other backup goes to every-other drive.