The 13 DNS root nameservers are known to every recursive resolver, and they are the first stop in a recursive resolver’s quest for DNS records. A root server accepts a recursive resolver’s query which includes a domain name, and the root nameserver responds by directing the recursive resolver to a TLD nameserver, based on the extension of that domain (.com, .net, .org, etc.
There are 13 types of root nameservers, but there are multiple copies of each one all over the world, which use Anycast routing to provide speedy responses. If you added up all the instances of root nameservers, you’d have over 600 different servers.