I find that 200 is a more comfortable number. This way, I have between 4 to 6 doff of each profession with different traits so I can easily do all high value assignments in one shot. Its why I can I typically get 2000+ dil every day from a single character just from doff assignments.
I haven't had a problem staffing the rare assignments even with lower numbers. However, I'm pretty choosy when picking up DOFFs. I look at how their traits contribute to the 'big picture'. Plus whenever possible I transfer DOFFs between characters so that each character has the DOFFs that benefits them the most. For example, if Doc Lee acquires a rare Engineer DOFF, he'll trade with Jho'dig for a rare Doctor DOFF.
My DOFF population is currently low for a couple of reasons, I've donated a lot of whites to the sb projects and I'm not running the full 20 assignments. Mainly focusing on Diplomacy/Marauding for now.
I wish the SB projects had been around sooner as I've dumped hundreds of DOFFs on the exchange, selling some for as low as 35credits.
I should check out Contraband farming also. I almost always run that assignment, so I probably have a little stockpile of Contraband. Where do I convert it to Dilithium?