According to Batch Data's analysis

Mom-and-Pop Landlords Still Dominate the Single-Family Rental Market
The overlooked power players in SFR: small landlords
Despite all the chatter online about Wall Street landlords and institutional investors, the single-family rental market is still overwhelmingly dominated by everyday owners.
According to a BatchData’s new analysis of every U.S. single-family home, 89.6% of single-family rentals are held by “mom-and-pop” landlords who own between 1 and 5 rental properties.
Some small mom-and-pop landlords view single-family rentals as a supplement to a 401(k)—or even as an outright alternative to one. Some are accidental landlords who chose to keep their starter home as a rental rather than sell it, while others jumped in during the Airbnb boom.

Of course, these figures do vary by market, with institutional players like Invitation Homes, Tricon Residential, and American Homes 4 Rent having a greater concentration in some Sun Belt markets like Atlanta.

In total, BatchData’s analysis calculates that there are 86.5 million single-family homes in the United States.
Of those 86.5 million single-family homes, 15.7 million are single-family rentals, according to BatchData.
Source: ResiClub