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?
Here is my quick lesson on Contraband farming.
Level a KDF to 50. Their marauding rewards plenty of contraband. I recommend only doing marauding assignments with a reward. If you do this regularly, you will max out marauding cxp before you know it.
Start out at the Klingon Homeworld. Go to Pi Canari Sector and take the marauding missions offered, and the first officer missions under your officers tab. Then go to Eti Eridani Sector, and do the same. You will get plenty of marauding assignments for farms, bases, refineries, etc.
Then make your way to Beta Ursae Sector, either by direct transwarp if you have it, use the transwarp to the asteroid mining, or cut through Orellus block. Here again take the marauding missions, paying special attention to the freighter missions. It seems the Ferengi freighter has the best payout.
You also get rewards of prisoners. Do the prisoner missions found under military, not the one you go to first city for. The payout is comperable, and you have a better chance of success.
I end up with anywhere between 5 and 20 contraband a day, depending on which assignments are available. And I haven't finished optimizing my KDF doffs for the necessary assignments.
Also, click on operations under your officers, and expel colonists. It's an easy 500 dil and is available once every day or so.