The Domain Name System root zone is served through 13 logical root-server identities.
These identities are traditionally named with letters from A through M, such as A-root and B-root. The number 13 is not a simple count of physical machines. Each identity can be supported by many geographically distributed servers using anycast routing, so users around the world can reach a nearby instance.
The limit arose from the size of DNS responses that could reliably fit in the original 512-byte UDP packet limit. The root-server system therefore uses 13 named identities rather than representing every physical server separately in the root delegation. Modern DNS extensions and operational techniques have allowed the system to scale far beyond 13 machines.
Root servers do not store every website's full content. They direct resolvers toward top-level-domain name servers, such as those responsible for .com or country-code domains. Those servers then guide the lookup toward the authoritative server for a specific domain.