Why is there only 13 Root Domain Name Server clusters?

Open this link and you will see the current root server list and corresponding IP Address:

At the same time, you will find that the root domain name server has only 13 IP addresses. Why?
Let's start with rfc1035 in DNS.2.3.4. Size limitsIt clearly stipulates:

Various objects and parameters in the DNS have size limits. they arelisted below. some cocould be easily changed, others are morefundamental. labels 63 octets or lessnames 255 octets or lessttl positive values of a signed 32 bit number. UDP messages 512 octets or less

Why is there only 13 Root Domain Name Server clusters?