Such objects (if that's the right word for them) affect space for hundreds of thousands — even millions — of light years around them.
Such power stuns the imagination. It is only through the infinite scalability of mathematics that we can begin to comprehend it.
Most galaxies have supermassive black holes at their center, but some do not. One reason is the supermassive black hole is slingshot out at high speed due to a sort of lopsided collision with another. But every galaxy should have at least some stellar-mass black holes which do not influence space for light years around them. Neutron stars are also stellar-mass remnants but not massive enough to form a black hole.