You could probably get away with one, though i don't know if the internet powers that be have any sepcific regulations about this. I use 2 even on small domains, just for fault tolerance purposes (really just incase I need to take a box down). Right now I have 4 nameservers for a 150+ node domain--2 win2k boxes for internal AD purposes, and 2 BNID boxes on my DMZ. As for using a private IP address, you should be able to use them so long as your firewall or router accepts and forwards packets for that box. All of my nameservers sit within a private range, but can be reached at an few different ips bound to my firewall's external NIC.