I see that almost all registered domains have at least two authoritative name servers configured (and for most of them, exactly two), and that using two servers provides redundancy against any kind of server failure. Technically there's no restriction on number of auth NS servers, so a single NS also works.
However, for some zones, there are already multiple addresses for one server configured, like ns1.qq.com (4 IPv4 addresses, auth NS for qq.com.). And for some other zones, the name servers are distributed using techniques like anycast, like ns3.cloudflare.com (one anycast IPv4 and one anycast IPv6).
In those cases, why would one still configure multiple authoritative name servers for a zone, when one NS entry can already provide redundancy against physical server failure?
Best Answer
In the early days of internet, registrars required you to have two DNS servers at different physical locations before they would register your domain. I don't know what today's requirements are, but I'm sure that historical tradition has carried on past the point where it was strictly necessary to provide redundancy. And a lot of the ones you see are probably the original NS pairs that were registered, and it just never changed.
It's also much simpler to do it that way than to set up anycast, hot failovers, etc.