Most landlords only own one or two properties, but most properties are owned by people with more than 3. Both these things can be true. And it can also be true

Most landlords only own one or two properties, but most properties are owned by people with more than 3. Both these things can be true. And it can also be true that though most properties are not run through such a system, only a very small number of properties are available at a particular time. And if such a system encourages more churn, then a much greater percentage of such properties would be available at any time, than the average. An effective monopoly could conceivably be maintained even if they only represented a fraction of the total market.