It's dependent on your situation. 

It depends how long you plan on living.

Obviously, a lot depends on your price range here and exactly what you're looking for. 

There are many routes to go when starting to find leads it all depends on your budget and experience.

Well, of course it depends on the individual circumstances, the amount of time comitted and the capital available.

or just explain as in a demand.it this 40k gross or Net.average rentals these days produce either negative cash flow or 100 to 200 to 500 a month positive depends on how much you put down. if you pay cash much higher.but lets do average 200 month positive and lets say you meant to ask NET cash flow to live on. then you need to take 40k divide by 200.00 and there you go you need 200 doors .

It depends where you are on your wealth and life ladder.

Depends on the area. 

Depends on rhetoric vs reality.

Depends on what you're trying to accomplish.