Thats difficult to answer:
1. question : If the ammount of your networker cfi Databases are exceeding 500GB I would think about a new Networker datazone.
2. question : Depends completely on your infrastructure.
If you need more devices (16 with network edition) in your networker datazone you have to purchase additional storage node licenses.
If you have I/O congestions you can implement storagenodes to balance the load.
With now affordable 10GB Ethernet infrastrucrure and new Backup-hardware, you can get a hell of backup performance out of a single intel box.
In some cases you should implement storagenodes to shape your maintenance window.
If the existing storagenodes are allways backing up or staging data, you cant patch them.