57 million, but that oncludes 17.6 million that are in third-party at-risk plans where Medicare is just paying a portion of the premiums (Medicare Advantage), not traditional "single payer" Medicare, so don't count in Medicare's negotiating power. So there's just under 40 million in traditional Medicare.

http://kff.org/medicare/fact-sheet/medicare-advantage/

You need to be careful with that definition. For prescription drugs, Medicare just pays premiums to private insurers under Part D. Following your logic, Medicare has zero negotiating power for those drugs.

The NHS is for the whole of the UK, which is 64 million people.