I think it should be 17M (12%), I tried plotting it in FRED: https://fred.stlouisfed.org/graph/?g=mGVF (the numbers themselves from the US Bureau of Census) The highest the US has gone recently is 15% vacant housing units, and it's been going down since the Great Recession. I think one reason why 12% is not so unbelievable is that if people move reasonably often, and it takes a certain amount of time to find a new tenant, then part of the vacancy rate will be proportional to the rate of churn of people moving in/out. For a single landlord, a 12% vacancy rate is about 1.5 months of rent missing per year, which is high but not hard to imagine.
Looking at that plot, it does seem like the vacancy rate correlates with housing bubbles quite well.
Looking at that plot, it does seem like the vacancy rate correlates with housing bubbles quite well.