https://www.nolo.com/legal-encyclopedia/is-your-rental-activity-business-investment.html
Example: Edwin Curphey, a dermatologist, owned six rental properties in Hawaii. He converted a bedroom in his home into an office for his real estate activities. Curphey personally managed his rentals, which included seeking new tenants, supplying furnishings, and cleaning and otherwise preparing the units for new tenants. The court held that these activities were sufficiently regular and continuous to place him in the business of real estate rental. (Curphey v. Comm'r., 73 T.C. 766 (1980).)
https://www.therealestatecpa.com/blog/when-rentals-qualify-as-a-business?