There are a few comments and notes on the Symantec knowledgebase which talk about setting NSPI to an unlimited number, removing throttling and limitations that Microsoft have built into recent Operating Systems to help Domain Controllers and Global Catalogue servers. Should you go for unlimited?
I personally don't make any changes in my lab, but I have seen several schools of thought on how this setting should be configured.
The article linked above suggests an alternative method to guestimate the value to configure. It is as follows (check the article in case this gets updated)
Number of Mailbox Archiving tasks = 2
Number of connection threads per task = 5
Number of Journal Archiving tasks = 2
Number of connection threads per task = 5
Number of Public Folder Archiving tasks = 2
Number of connection threads per task = 5
Task Controller Service = 20
Storage Service = 32
Shopping Service = 20
PST Migrations = 20
Total 10+10+10+20+32+20+20 = 122