> I’ve seen an estimate of 20,000 for SF proper.

Even if that was added to the vacancy rate, that would still leave SF with around the national average (~7%) vacancy rate, and that's without counting the off-the-market housing anywhere else in the average (and not double counting actual on-the-market vacancies everywhere else, either.) Anyway you cut it, SF has a low vacancy rate/rate of empty homes.

Yeah, but 2% to 7% vacancy rate can mean the difference between rents that are $2000 per month and rents that are $4000 per month.

SF added 2,000 jobs in 2019 alone. If those were put on the market even in a huge lump sum, there's no way it would cut rent prices in half. Maybe it would keep the prices from rising for a year or so?

It's low no matter what measure you look at.

You'd also have to consider all the people that would be happy to rent out their in-law but decide not to because of rent control.

Not saying it would solve the housing crisis, but adding another 20,000+ units onto the market would certainly help.