There is no maximum number clinics that a physician can own. Restrictions would be related to the delivery of medical services at the clinics, rather than ownership of the clinics themselves. For example, if the clinics were staffed with NPs, the state may have a maximum number of NPs that the physician could collaborate with. Even then, though, the owner could hire a different medical director for each clinic.
So, yes, an MD could own 50 clinics. I can't speak to the annual income. It would depend on many factors, such as what services are being offered and the volume of patients.