Good luck with that.
My prediction is he gets less than half that $500 million on second homes of the rich and his budget is a fantasy in practice.
If I were really rich and had a second home that was going to be taxed, I'd "sell" it to a corporation I own that was in another state and then "rent" it to myself when I stay there instead of owning. Now, Mamdani gets ZERO tax benefit as the second home is owned by an out of state corporation. Sure, the other state gets some small tax benefit (because I picked a low corporate tax state) but that's nothing compared to dodging Mamdani's tax.
The rich didn't get rich by being idiots. But any idiot can be a tax the rich socialist. Socialists always wonder why their tax the rich schemes fail...