I'd wager it's a multiple. A proportion of licensed psychologists are doing little to no therapy, so it drives that number down. If we hold up the left over number against the numbers of master's therapists/LPCs, SWs doing mostly therapy, and MFTs, I'd imagine they have 3X+ our numbers. Private practice numbers are harder to get, but in my non-VA/non-AMC hospital experience, psychologists doing therapy vs. the other group was about a 1 to 10 ratio, at least. At my last placement, we had 1 psychologist in the OPMH unit, and about 15 other therapists. And there was 1 psychologist who did consults on inpatient units against a larger ratio of other therapists.
But, back to the point of the numbers. If you are solely looking for psychologists who take medicare for OP private practice work, you may be in for a wait, though almost all psychologists who work in a hospital system will take it. But, if you are also considering master's level folks for therapy who take Medicare, you should have an easy time referring out in most places.