Was a black hole formed by only one star or many stars? And can a black hole be formed by other materials (non-stars). Although I know that a black hole can be formed by gravitational collapse of a massive dead star, I'm not sure whether a black hole can be formed by collapse of many stars.  
 A: Recently with the LIGO experiment  two black holes were seen to merge into  one back hole.
It seems that  LIGO also may have detected two neutron stars merging into a black hole.

The object resulting from the merger is about 2.7 times the mass of the Sun. As the object continues to be studied, evidence is mounting that what was created in the process was a low-mass black hole (if true, this would represent yet another new class of black holes discovered by LIGO). Continued observations over the next several years may confirm or refute this notion.

Future observations will show.

The black hole responsible was Sagittarius A* (pronounced “Sagittarius A-star”), the supermassive black hole at the center of our Milky Way galaxy. Astronomers think that most large galaxies like the Milky Way should have supermassive black holes in their centers, but it wasn’t until the past couple decades that they had compelling evidence that Sgr A* is our supermassive black hole.

There are  black holes in the center of galaxies, but whether it is a black hole eating a star, or two stars simultaneously merging into a black hole , I think it is a matter of modelling  and trying to find confirmation for the model.
