There isn't a way to specifically limit the number of files in a cache. You can keep forcing expiration to use a smaller and smaller percentage of the available space -- but leaving empty space in a cache filesystem is wasteful.
If you add a new cache filesystem, newly loaded files will go there first, as CMOD tries to distribute loaded files evenly across all cache filesystems.
For the record, you can only have ONE "retr" directory in your cache filesystems, and it must be the first cache filesystem. Adding more cache filesystems won't reduce the size (or number) of links in the retr directory!
Could you clarify the problem you're experiencing? Are you running out of storage space (gigabytes) or are you approaching the maximum number of files that can be stored in a filesystem (inodes)?
-JD.