I'd rule that wild shape hit points do not stack, according to the general rules about stacking similar effects.
So, yes, a 20th-level druid can change shape each round to replenish their bonus hit points, but they couldn't just sit there racking up hit points between combats and exceed the normal maximum of a CR 6 beast.
So, yes, a 20th-level druid can change shape each round to replenish their bonus hit points, but they couldn't just sit there racking up hit points between combats and exceed the normal maximum of a CR 6 beast.