I hope to some day have this problem. I set a goal of having 10 SFR by the time I retire in 8 more years. I looked at what my Social Security, pension, 401K, and Roth would bring in and with 10 rentals we should be well above the income level we have now. I figure we won't be rich but we shouldn't have to worry much about money when we're retired. Buying property is addictive and we might change our goal as we get closer to it. I could see expanding this to 20 or even 30 properties or even looking into some MF properties at some point. What's exciting it that just a couple of years ago I didn't even think where we are now was even possible. Who know where this will end or what will be possible a few years from now if we keep going. This is why I love doing this stuff. You can do as much or as little as you want. If you want to have an extra $10 or 20k a year coming in or if you want $100-500k coming in, it's up to you. I only wish I would have started doing this when I was younger.
My mortgage broker and her husband have 3 rentals and they have no plans to expand. It brings them some extra money and they use part of it to pay for a place they bought on the coast in Mexico.
My wife's former father-in-law has 5 SFR rentals and he's in his late 70's. He just bought 2 houses last year and uses the money from those to help his son pay for private school for his grand kids.
So when to stop buying is a tough question.