I only have 8 Boffs because I like having a limited number of Boffs for immersion purposes.
It's funny, because I do the opposite. As I collected BOs through leveling up or from the exchange if I wanted particular traits or better BOs, I gave each of them a role on the ship. After I filled out the chief officers (engineer, science, etc.) I ended up with MACOs, chief xenozoologist, stellar cartographer, and so forth. Of course, now, with the DOff system, that's not as relevant for immersion, but they all have a place on my ship.
I do have a core "senior staff" but some of them aren't used in lieu of "junior" officers with better traits or skills. So I guess I don't take immersion so far as to interfere with min/maxing, but it is something I think about.
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I came to the conclusion that 9 BOffs per toon were more than enough. I train 3 of each class. I had tried fewer, but there are a number of ships which require 3 of one class. 12 is just flat out too many, imho. The more BOffs you have, the more apparent it becomes that we need more class skills...