There is only one name server for your domain.
Only having one (1) name server leaves you vulnerable to that name server failing and taking down your website. It is recommended to have at least two (2) name servers for your domain to provide failover capabilty/backup in the event one name server fails.
RFC 2182 declares that The Domain Name System requires that multiple servers exist for every delegated domain (zone).
A major reason for having multiple servers for each zone is to allow
information from the zone to be available widely and reliably to
clients throughout the Internet, that is, throughout the world, even
when one server is unavailable or unreachable.
Multiple servers also spread the name resolution load, and improve
the overall efficiency of the system by placing servers nearer to the
resolvers. Those purposes are not treated further here.
With multiple servers, usually one server will be the primary server,
and others will be secondary servers. Note that while some unusual
configurations use multiple primary servers, that can result in data
inconsistencies, and is not advisable. [RFC 2182]