18446744073709551616 is 65 bits. Let’s simplify, you're trying to guess a number in 2^64 You can’t guess in parallel. Reasonable constraints on the server side (i.e. limit tries on the combination/per hour before suspending ssh for a while) may have been implemented.

I’d say cracking that is… Unfeasible.

That also assumes you know the existence of a server on which there is ssh under an unknown combination of port knocking of length 4.