My number was about $45,000 per year in 2014 with 2 years of living expenses in the bank. This would be enough to meet our basic needs. That's the year my husband and I turned 40. He was burned out at his job - with daily headaches, missing the kids' events, tired of being taken advantage of, no time for a life. I had been flipping houses mostly by myself and managing 10 apartment units. I started the "Free Mikey" fund, and we calculated the number of units we would need to get to $45k in rental income, so we bought 10 more units that year. He quit his job and started a consulting business. In 2015 he made 60% more consulting than he had ever made working for someone else.
Now we have 53 units and have done 1 or 2 flips per year. We've far surpassed our goal and have added new ones. Because managing the manager was such a hassle, we self-manage, but I look specifically for low-drama tenants and do a lot of preventative maintenance. We've decided to stop flipping houses in this crazy labor market since we don't want to do the work ourselves and don't need the money. He still does consulting work when he wants to.
Your question is valid because meeting our goal was much easier once we had a number. It's counter-intuitive, but we had to spend money to buy more property in order to have more money. We're trying to slow down a bit and enjoy life. Spending money on depreciating assets - like campers, cars and boats, is counter to what we spent years building, but we really need to have more fun! Read the Parable of the Talents to see why it may be hard for some people to stop investing.