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The mass of the central black hole is a tiny fraction of the mass of the galaxy, it might be more correct to say that the objects in a galaxy orbit the common mass (including dark matter) of the galaxy.

For example, the black hole at the center of the Milky way is estimated at 4.1×10^6 M (solar mass) and the total Milky Way mass is estimated at 0.8–1.5×10^12 M (figures from Wikipedia).

Some interesting articles found while digging for these numbers:

https://ned.ipac.caltech.edu/level5/Merritt/Merritt1.html

https://academic.oup.com/mnras/article/420/4/3621/975383

https://academic.oup.com/mnras/article/414/3/2446/1042117

Planets orbit around the center of mass. It's coincidental that the center of mass is (at least in our case) at the sun's location, because the sun has most of the mass.

I believe the current theory is that most (all?) galaxies have a supermassive black hole at their heart.

Yeah, or so I heard. What's the cause and effect order for galaxies and their corresponding supermassive black holes?