If you just mean for DNS, you need 1 per nameserver with a minimum of 2 nameservers.
If you're buying a reseller your hosting company may have it set up so you can use theirs and you don't need to buy your own IPs.
If you're the main server administrator.. you should know .. so I'm assuming your not.
Only 1 IP address?
You really need 2 IPs, the nameservers on a server shouldn't have the same IP and there should be a minimum of 2 nameservers. You can use the server's shared IP for one nameserver and then an additional for the second, if you're limited.
Do you have this box with a datacenter? Or.. at home?