Yes, one is all you need unless you plan on running ZeroShell as a router/Firewall. I’m new to this as well and initially ran ZeroShell solely as a RADIUS server but then recently set it up to act as my DHCP Server as well. I have it setup next to my FreeNAS server and my LAN is running smooth as can be expected.
Let us know how you make out on the HD install. Boots a lot faster and you wont have to hear that whiny disk spinup sound.