I haven’t done much research on necromancy, because I have many things to learn and can’t learn them quickly enough. Here are some questions I have:
Does necromancy limit itself to your ancestors or can you use any dead individual like a celebrity?
How much can you indulge in necromancy before it destroys your body?
Is the death current easy to control or doesn’t take lots of training before you can stop it from harming you?
Best to start either with your ancestors, or learn psychopomp work from a core shamanism teacher (like the Foundation for Shamanic Studies) who will teach you to work WITH the dead instead of trying to command them (“use” them etc) and thereby, piss them off.
Yes we’re all badass black magicians, but why be on egalitarian friendly terms with demons and yet treat dead humans like slaves? JMO!
How much can you indulge in necromancy before it destroys your body?
Learned correctly (see above) working with the dead doesn’t destroy your body.
I have literally no idea what crazy shit people do that causes this to happen to them, but then I don’t understand why people sniff glue or watch X-Factor, so, whatever floats your boat. 
Is the death current easy to control or doesn't take lots of training before you can stop it from harming you?
You need to be competent at energy work to be able to backfill with healthy living energy if you spend a lot of time with deceased persons. Balance in all things is important.
Lots of people are all “grr - death current - black dark blackness” and I’m sure that’s very nice, I personally have had some excellent success working with Buné on this, if you search my posts mentioning her you’ll see them, I can’t be bothered to do it and then format links, soz - been a LONG day. 
Well here is a common-sense answer (but knowing good sense isn’t common =)… First off when it comes to Death Energy, like drugs it is already a Naturally Occuring Element inside your body. So drumming up death energy within yourself would effect things like the Rate of Decay… is it any wonder that people into such practices would have a morbid/depressing outlook?
People generally do magick to create some form of change in reality, so there is a Breakdown Process meaning there is a very Minute Element of Death energy within any given situation. If you have excessive amounts of certain types of energy it is best to have something that they are all directed at, otherwise it could mess with alot of things.
I am not quoting just Death Energy here, but actually raising any sort of energy causes a certain amount of Death Energy to be present in the form of your body breaking down, because your Physical Body possesses a Finite amount of energy reserves that is being “burnt up” to produce magickal energy. You may notice this when your body energizes and heats up when doing certain things. So when living your life it is best to be very Hyper-Aware and control every moment and directing where your energy goes to Optimize it for yourself.
To Lady Eva’s comment on working with the Dead doesn’t destroy your body that can be mostly correct. From my POV regarding energy usage it can be considered a very very small death from an energy loss standpoint as generally the more Active in anything you do in life your always using up energy. Don’t believe me? Increase your positive energy levels and see how many spirits (which includes humans) want to hang around you. =)