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.