How much time do you believe it takes to "manage" those 20 houses? A month a year at most.
(Source: have friends with low 10s of houses).
* Property advertisement - Preparing material and advertising for 1 property can be more expensive than multiple properties
* Material for fixes - Purchasing material for 1 property can be more expensive than multiple properties
* Vacancy - The cost of vacancy for a single rental is higher than having multiple rentals. The other rentals may off-set the cost of the vacancy lot for a period, without requiring external cash flow.
* Experience - Multiple properties will give you the opportunity to experience different issues and allow you to take proactive steps to diminish certain risks.
When I first started buying houses to rent, I had a rental unit in a LCOL area, and I spent a lot of time dealing with tenants and rent issues. It was a constant problem, so I sold the house. I might have broken even on the venture.
I ended up with a handful of starter homes near nice schools, and had a much higher class of renter as a result. Far less work, vastly fewer headaches.
I also had a FT job, so didn't have the time or energy to chase down rent checks.
A few weekly calls during the workday. A few evenings or weekend daytime meetings with prospective tenants or maintenance staff each month. So, ~$1 to 1.5k per hour of work. Not a bad paying side gig.