As far as I am aware there is no theoretical limit that they cannot surpass, but of course they are limited in practice by the available mass, and how fast they can consume that mass; a lot of mass ends up orbiting, instead of falling into the black hole. So black holes such as the one mentioned in the article are expected to be rare.

Once they reach a certain size the stars in the closest stable orbit are no longer ripped apart by tidal forces, which means that they do not grow significantly beyond this.