The innermost stable circular orbit around a non-rotating black hole is 3 Schwarzschild radii.
For a Schwarzschild black hole, which has no spin, the ISCO lies at a coordinate radius of 6GM/c². The Schwarzschild radius is Rₛ = 2GM/c², so the ISCO is 3Rₛ. This is the smallest circular orbit in which a massive, non-spinning test particle can remain stably orbiting.
The ISCO is not the same as the event horizon or the photon sphere. The event horizon is at 1Rₛ, while the photon sphere is at 1.5Rₛ and supports only an unstable circular path for light. Between the ISCO and the photon sphere, massive particles can have circular orbits, but those orbits are unstable.
The ISCO is especially important for accretion disks. Gas spiraling inward can orbit stably outside this boundary, but once it crosses the ISCO, it rapidly plunges inward. Rotation changes the result: for a spinning black hole, a prograde ISCO can move inward, while a retrograde ISCO moves outward. Thus, the value 3 applies specifically to a non-rotating Schwarzschild black hole.