Beta cells are destroyed only during type 1 diabetes and are not associated with dietary choices.

During type 2, your body becomes resistant to the effects of insulin produced by the beta cells and they go into over drive providing insulin. Eventually, the beta cells become over stimulated and reduce effectiveness, but they don't die.

There's no reversing type 1. You can have remission of type 2 and your beta cells return to normal. Neither outcome depend on "how many are left