Find your sales rep and ask for a copy of the license/invoice  or ask whomever does the bills for your department and ask for the list from the last time you updated the contract.

Symantec has a licensing portal which has all these information. Generally, when a client is allowed to draw down a license(from this portal), they have the option of getting it for either a single quantity or multiple quantities.

For example, if your master server has 50 clients in its policies, you generate a single license for a quantity of 50 and add to the NetBackup Master. You can also draw down 50 separate licenses, but there is no way you can relate it each client (or DB Agent). 'bpminlicense -summary' will only give the type of license.

The best way, as mentioned by J. Hinchcliffe, is to contact your Sales Rep or a Symantec Sales guy.

Call Customercare and request for an Install Base Report of all of your licenses, they could look up for your licenses using your Company Name and Address and be able to check the Licensing Portal Account details of your company.

Hope this helps.

Netbackup Licensing Faqs
http://seer.entsupport.symantec.com/docs/314816.htm 

Admin guide - Download and check chapter-12
http://seer.entsupport.symantec.com/docs/290203.htm 

Licensing Portal
https://licensing.symantec.com/ 

Licensing Portal guide
https://licensing.symantec.com/helpdocs/LicPortal_LicMgmtUserGuide.pdf 

Customer care numbers
http://www.symantec.com/business/support/assistance_care_pop.jsp

Tech-support contact
http://www.symantec.com/business/support/contact_techsupp_static.jsp 

Basic Recommendations:

1.  Make sure all your licenses are in the Symantec licensing portal under your customer number.
2.  Depending on how the licenses were purchased, there can be multiple customer numbers for the same company.
3.  Customer numbers are tied to an email address.  Ideally one company should have one customer number. 
4.  If licenses belonging to your company were tied to a email address of a former co-worker, you may not be able to see those licenses in the portal. 
5.  For the license portal login, use an email alias, instead of a personal email address, so that multiple people within the company can view the licenses.
6.  Call customer care and have them verify if there are any other licenses belonging to your company that are under another email address, or customer number.
7.  Call customer care and verify which licenses are still under active support.  Active support is required for all licenses in use.
8.  Print out a hard copy of the licenses for reference.  Liicense keys will change when they are upgraded.