There is no recomended ability.
But the basic sailor should have at least 100 vets and full (or close to full) experts.

IJN AA gunners (late classed) don't really need to be overvetted (more than 100 vets) to work but it depends on their base.

Most main gunners reach the (mobile) caps with 100 vets and (nearly) max experts at around lvl 70.

Scouts - you can either ghost him to save weight or use the standard 100 vets max experts. It helps them resist AAW but if they get caught into AA range or fighters attack them then even 200 vets won't save them.

Engineers/Repairs - I dunno. I seriously dunno. Personally i keep 150 vets on every sailor and i'm still far from the repair cap and my 4 engineers (base 11) had to be around lvl 110 to speedcap SY. 

For IJN gunners, as long as your base isnt total crap, 100 vets should be fine to reach the caps.

For Engineers the only cap is OH speed which is constant with all ships. Overheat time however is not capped so the more ability the better. It is better to keep your engineers ability close to eachother though.

There is a cap on repair rate that is constant with all ships but only UK can reach it relativly easily so again the more ability the better.

For Scouts (and all aircraft) defense ability is uncapped. Scout defense is controlled by the aircraft ability.

Alright thank you guys. :)

BTW what is the amount of MAX Experts? About 400 Experts? 

Max experts? Or do you mean max vets?

Max vets are 40%, Max experts I do believe would be a chief gunner with no vets @ level 120 :)

Still dont understand what you are asking. Try reading this and see if you still have questions: http://www.nf-guides.com/sailors.htm#b2

You cant add vet packs if you have 100 vets already.