How many yrs does a felon on probation in the state of ca get with possession of a firearm and amunition?

Difficult to answer - too much is unknown. You can be pretty sure that they will be returned to jail/prison to serve the remainder of their original sentence.

In the meantime they will be charged and tried for the firearm possession offense, and that sentence will probably be added onto the top of their original sentence.

If the felon was charged under the federal statute they could be imprisoned for up to 15 years on the firearm charge alone.