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But it means there is probably more they can't see

>By extrapolating from the properties and distribution of these binaries, the team estimates that there may be 300-500 low-mass binaries and 10,000 isolated low-mass black holes surrounding Sgr A*.

Every object in the universe attracts any other object. My initial impression was : not only one ?

well it almost seems like there are only 2 options:

only one and so many we can't possible try to count them all

This is just about the center of our galaxy though. There are billions of other galaxies in the observable universe, most of those I assume also have black hole(s) in their center :)