However, you could use some descriptive phrases depending on the context:
* A pack of dens: This works if you are talking about animal dens.
* A cluster of dens: This emphasizes the close proximity of the dens.
* A collection of dens: This is a more general term.
* A network of dens: This suggests the dens are connected in some way.
Ultimately, the best collective noun to use depends on the specific situation and the desired effect.