You don't have to use nameservers in pairs. The reason they are supposed to be in pairs is for redundancy.
The pair of nameservers should really be in seperate locations as well. In truth two nameservers on the same server doesn't achieve the purpose of havning two.
Having 5 is not that odd at all, just uncommon. These 5 nameservers may be located at 5 different datacenters throughout the world providing DNS services to the websites in the event one nameserver is down.