Virtually none. Property taxes and upkeep guarantee that that's a losing strategy in the long term. Despite the covid-era spikes in real estate prices, house prices here aren't going up fast enough to justify buying houses and keeping them empty in the hopes of selling them for a profit later. Empty houses here are empty because their owners only live in them for two months a year, or because they're in bad condition and are more or less abandoned.

Back in 2005 people in some real estate markets like Arizona and Florida were buying houses and keeping them empty because prices were skyrocketing, they knew they could sell the house in a year for a big profit and the good times were going to last forever. Of course, once the bubble popped in 2006-2008 they all got caught by the recession and those empty houses got foreclosed on. The market conditions and easy financing that made that kind of speculation possible haven't been repeated yet.