That depends on several factors:
1 What missions are in your deck?
2 What is the current state of the META you intend to face, regarding dilemmas?
3 How big is your draw deck?
4 Which skills can you bring out reliably before or during the game (so no draw
luck involved in getting them -- only some effort in protecting those personnel)? This factor will modify the below ideas.
Others will hopefully give you more tips!
But in my decks of between 40-60 cards, I tend to include at least 8 or 6 of each primary personnel type. Then, 5 or 6 of skills you'd need 3 distinct personnel with to pass certain META dilemmas, and the secondary personnel types (which you don't often need 3 of -- which depends on the META).
That's if I can get lucky with skill boxes. I might be tolerant here or there.
Then, at least 4 of each skill you need for missions. (Non-negotiable for me, except when can be brought out and
I try to have all remaining skills at least once; for fun/diversity, and the off chance you face 'that one dilemma'. (I often played with non-META dilemmas, just for fun and surprise -- so it does
happen.) But that's probably not very sound advice.
And overall, I have a peculiar way of building my decks... so don't follow my suggestions blindly.