I run about 40-50 skeletons and have never needed more than that. Mist of the time, it is better to just field a large unit of skeletons and heal them with all your IoN instead of growing the unit with the IoN of your one necro. The reason is that necro's IoN is the one they dispell, so you can't benefit from MotD. What they should of done, automatically let necromancers raise skeletons beyond starting size and then let vampires buy MotD as a power. That way all your casters could have it.
Any rate, MotD isn't worth the points or hassle.
With that said, zombies are another matter. You should have a minimum of 20-40 extra zombies beyond what you field. I personal own about 150 zombies and have successfully summoned all of them onto the table before. Between fielding a 20 zed unit, casting 3 or more IoNs and using raise dead, then rerolls from corpse carts. Once you get good at using those things, you can start needed lots of those guys.