Currently? at least Vancouver, and possibly Toronto. I don't know about other markets. It's roughly 4.3% of Vancouver's housing stock[1]

Toronto has roughly 23% [2] of its for sale stock vacant, unfortunately there have been no openly published numbers on total vacancies.

[1]https://www.cbc.ca/news/canada/british-columbia/empty-homes-... [2]https://www.narcity.com/ca/on/toronto/news/a-shocking-number...