Imo, the ethical limit is 3.
- To live in
- For additional income from rental, retirement security etc.
- A country or seaside house for weekend/summer getaway
There's no real reason to own more property than that. If you have extra money to invest put it in actual business. Into new housing construction for example you get quite a return on that, and it doesn't make you unethical.
Edit: This also applies to companies. Actually companies shouldn't own any housing at all. Selling at a profit that's acceptable. Owning it as an 'investment' - absolutely not.
TIL Woodrow Wilson was a huge racist.
In Europe there are quite a few streets named after him, due to US joining WWI under his leadership.