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I am not sure I understand what you mean by "estimate of the number of medium-sized stars (sun-sized) that would fill it." Fill what?
'Solar mass' is a standardized unit of mass roughly equal to the mass of our sun. From Wikipedia: "The solar mass is a standard unit of mass in astronomy, equal to approximately 2×10³⁰ kg."
If by "fill it" you are referring to the black hole at the center of the galaxy, nobody really knows what goes on inside of a black hole. By definition, the event horizon is the limit of observability; what happens inside that radius is unknowable at least based on present physics. The average density of a black hole, using the event horizon as its radius, actually decreases with increasing mass. Logic would say though that the density at the center of the black hole (the singularity) would be extremely high, and probably greater as the mass increases. But we cannot know this for sure.
Couple interesting sites:
This last one calculates the mean density of a black hole using the event-horizon-as-radius method. (Apologies if you already know all this; hopefully someone else will come upon the discussion and find it interesting 😎 )