If you only like to play ss6
Then 1 BO, 1 sonar, 1 planeman, 1 torp. and 5 engines.
If you like to play other ss,
you need one more torp and gunners.
As mentioned above, you may train your crews with ecl, pca, and pbb/ebb. Then you need gunners, scout, and maybe aa (I dont like aa)
In term of training,
I like use engines (11 or 12 repair/ 11 or 12 engine). Definitely, the hero, 90 lvl neutral, or legend sailor is the best, while they are not tradable. For engine, just class on time and dont over vet them.
I am using a bo 15pot/12rep and like it.
Planeman elite pot sailor and class on time.
My sonar is 15pot/12rep and late class, as said by many ppl that ability for sonar is capped.
torps(11 torp/12rep). I classed them on time. You may consult with more ppl. because torp ability is capped soon.