You can have small moons in the two stable (L4 and L5) Lagrangian points of a main moon, like the Saturn moon Dione's companions Helene and Polydeuces, or Tethys' Telesto and Calypso.
If you want more than three moons, you can organize them in a Klemperer rosette, although this type of configuration is not stable.
Another option is to have a huge number of small fragments orbiting inside the
Roche limit, preventing them from forming a larger object. In a way, the rings of Saturn can be thought of like many small moons orbiting in the same orbit.