https://thenextweb.com/science/2021/01/22/new-physics-theory-postulates-the-existence-of-slabs-stupendously-large-black-holes/

At the center of the Milky Way lies the biggest object we can be sure exists: a supermassive black hole (SMBH) four million times more massive than the sun.

Physicists believe these huge singularities can reach even greater sizes if they’ve eaten a big enough galaxy. The biggest estimates are somewhere in the range of 10 billion times more massive than our sun, but things begin to top out shortly after that. At least, that’s the conventional wisdom.