They might be fewer than their cousins but I don't think they would be so few that they couldn't crew a couple of battleships. Not at this point in time anyway. Besides which, they could always just do with having their experimented monkeighs and other races on as crew.
oh, i don't know that they're meant to be necessarily so much
fewer in number than the other branches of the Eldar 'tree'-- the sheer rate of attrition in DE society would indicate a far higher birthrate, for starters, and Comorragh is described in terms more apt to a "hive" world than a single craftworld/city. but they are a lot more
divided amongst themselves, into relatively small, mutually antagonistic kabals, and the concept of enlightened self-interest doesn't really figure in their society. the total assets of Comorragh, in military terms, probably exceed those of all the major craftworlds combined; but each craftworld functions as a cohesive, self-sufficient arcology several million strong (they have to, in order to survive), whereas DE kabals would tend to max out at a few thousand, and are constantly in flux (in fact, the larger and more powerful a kabal becomes, the more the inevitable entropy and internecine conflicts accelerate its demise).
the idea of slave crews is a neat one. DE technology in general seems to be a lot less psychically-oriented than other Eldar stuff, so they might well be able to train slaves to basic shipboard operations (damage control, maintenance etc.) slaves still need guards and overseers (and food!) though...