When a druid uses Wild Shape to transform into a beast, how many Hit Dice does it have?

Suppose that, for whatever reason, my mid- to high-level druid is in a party that has decided to take a short rest. Instead of spending a use of their Wild Shape to assume a new form with full hit points, my druid decides to spend the short rest in beast form, and spend some of its hit dice on healing.

How many hit dice does the Wilde have upon being transformed into? All? None? Some?