Excluding the central nervous system, vascular and respiratory systems, what parts of your squishy insides could you without?
apart from the spleen, the apendix and one kidney we kind of need it all, unless theres a mechanical/technological replacement
I'm not a doctor, but the womb, blind intestine, testicles and urin bladder can be removed if it's okay to constantly pee on the floor. If removing stomach, intestines, liver and kidneys, the nutrition has to be a pure liquid entering the blood directly and possibly a machine is needed to purify the blood after a while, but then the victim is probably dead from internal bleeding. Any sugar in the diet has to come with insulin to break it down. The heart can be replaced with a pump, but a killer wouldn't bother to pay for that super expensive wasted surgery. Lungs are practically impossible to remove without killing the victim, but replacing its function should be possible if someone filled them with a liquid and somehow kept oxygen in a huge amount of blood by connecting to another victim. It's probably more a question about how many organs can be removed, how much it would cost and how long the victim would survive before the body had enough with trying to heal and fight infections. Might flatline before half are removed with so many things that can go wrong.
This question depends on the functionality you desire. You could remove the frontal lobe of the brain in a lobotomy. The appendix, of course, but that does have digestive functionality. Gall bladder, but grease cant be processed anymore. Testicles, but no kids of course. Bladder could be removed, but needs a false bladder installed or they would go septic. One kidney or one lung could be removed without much difficulties.
My niece is missing ~40% of her intestine and functions mostly normally. She has to be careful with her diet, but it's not a major imposition, or wouldn't be if she bothered eating healthy.
@DarkPen14 Google is your friend for this kind of a research-related question. For example: https://www.google.com/search?client=firefox-b-d&channel=trow&q=what+internal+organs+can+the+human+body+live+without? The answers to these kinds of questions are easily found by Googling the topic. For this one, I Googled: What internal organs can the human body live without? Bingo. Tons of information, some of it from medical resources. You should be able to find whatever you need.
Jesus, you scared the hell out of me, @jannert. I got the notif that a mod tagged me and my first thought was "what did I do this time?" Jokes aside, thanks, y'all
Liches is the plural of lich, a magician who has voluntarily entered a state of pseudo immortality through a twisted form of ritual sacrifice.