[kuang_eleven]

> How many empty houses are there in the US, millions?

About 16M units unoccupied [1].

> How many luxury condos are being built

Quite a few! New construction is almost always targeted at the high-end, which has been true throughout history. Low-end housing, outside specific government projects, tend to be the high-end housing of the past

> and sit unoccupied?

Very, very few. The idea of luxury housing being built only to sit empty for some unspecified "investment" reason has been a persistent myth that has never been backed up by data.

1. https://data.census.gov/cedsci/table?q=DP04#

[folkrav]

So basically, what you're saying is there are millions of unoccupied units, which should be older, now lower-end units, but people still can't afford to buy them.