The 512bytes limitation of UDP payload

512 bytes are too small for IPv6 contents (AAAA/A6)

Calculation on the root zone
NS records + A/AAAA/A6 "glue" records

    RR        512
    -------------
    A          15
    AAAA       11
    A6         10
    A + AAAA    8
    A + A6      7

Assuming that question is "."
Currently, 13 root servers are operational.