According to the French Wikipedia article on the town (commune) of Montrésor, quoting French government statistics, nearly one in five logements (housing units) in the village is vacant. Another 20% of the village's logements are occupied only seasonally or occasionally. There are 280 logements in all.
In other words, only about 60% of the village's logements are occupied on a permanent basis as somebody's résidence principale. In the rest of the département (the Indre-et-Loire, pop. 600,000), which is centered on the city of Tours, the percentage of logements that are résidences secondaires (second homes) is only 4.4%. The number of second homes as a percentage of Montrésor's total housing stock has increased from 9% fifty years ago to 21% today. Well-to-do people who live in Paris and other cities famously have maisons de campagne (country houses) to which they can retreat for vacations and weekends. Often these houses have been inherited.
For comparison purposes, in the village where we live (pop. 1,100) there are more than 600 logements and, as in Montrésor, nearly 20% of them are occupied only seasonally or occasionally. However, only 3.5% of the logements here are vacant, compared to nearly 20% in Montrésor. In the "hamlet" or neighborhood we live in, there are nine houses and only five of them are anybody's principal residence. The other four are vacation/holiday/country homes that are only occasionally occupied.
One major difference between our village and Montrésor is the land area each occupies. Montrésor is about one square kilometer, while our village covers 32 square kilometers (12 sq. mi.) of territory. In what I consider the Saint-Aignan "metropolitan area" — four towns/villages covering about 80 square kilometers (30 sq. mi.), pop. 8,300 — there are about 4,600 logements and about 10% are second or vacation homes, not résidences principales. In a similar area of 83 sq. km. around Montrésor (three other villages included), there are about 1,500 permanent residents. The Montrésor area is really sparsely populated.