The ZEBRA RZ package supports a maximum of 65000 records per file. As FATMEN uses RZ to store the catalogue information, the same restriction applies to FATMEN.
The default record size of a FATMEN catalogue is 1024 words. If a larger record size is used, one may store more entries, although there is an overhead concerned with directory records, which always take an integral number of records.
For efficiency reasons, one should not store too many entries per directory. Experience suggests that a few hundred to one-two thousand entries is a suitable number.