Number of IPv6 addresses: ~340 undecillion [1]
Length of string of words: ~8.4 [2]
Probably closer to 8 as what3words likely doesn't cover the full 3-word-address-space of the English language.
[0] https://en.wikipedia.org/wiki/What3words#cite_note-16 [1] https://en.wikipedia.org/wiki/IPv6#cite_ref-rfc2460_2-2 [2] https://www.wolframalpha.com/input/?i=log+base+38485+of+340%...