Once all nodes are crit., all you need is 5 crew. You may have to consult the wiki to make sure that you cover the rare rewards.
For Saturday challenge you only need four crew. For Sunday’s challenge you need to add one crew to guarantee success on last epic mission I think 5 crew choices are available. On one account, I use Klingon Dukat on the other Troi.The four for Sat. challenge are Saavk, B’Etor, Ezri, and Torres.
Saavik is used for SCI/CMD, B’Etor DIP/SEC, Ezri SEC/MED, Torres for ENG.
These four can garner all rewards and guarantee success. On all missions except Sunday’s epic Of Two Evil.
So if you are not worried about that one mission on epic, all you need are those four.
Great, I’ll add this to the OP, thanks!