Depends on the system. In this case it seems like retries are possible after a failure, so two is sufficient to detect bad data. You need three in real time situations where you don't have the capability to go back and figure it out.

Spot on. Very often doing an occasional (very rare) retry is acceptable.

Sometimes doing the same processing twice can be also a way to implement safe(r) rolling updates.

Two hosts is efficient. Do it twice on two different hosts and then compare the results. If there is a mismatch, throw it away and redo it again on 2 hosts. A total of 4 computations are needed. But only if the difference really was due to bit flips, the chance of which are exceedingly rare. In all the rest of the cases, you get away with two instead of three computations.