Definitely a licence per filer.

"One NetBackup for NDMP license is required for each NDMP-enabled NAS 
requiring protection.

Licensing is per NDMP NAS (host) regardless of the configuration (local, 
3-way, or Remote NDMP) or number of data movers. For example, if a 
customer has 5 NDMP-enabled NAS devices that require backup or restore 
services from NetBackup, 5 NetBackup for NDMP licenses would be required. 
In addition, NetBackup for NDMP is licensed per physical system, 
regardless of the number of data movers
configured on the NAS.
"

William D L Brown
Ext Tel: +44 1438 762681 Fax +44 1438 762336 Mobile: +44 7717 800241