I'm writing a story where two lovers meet then I having one of them die at the end. I am curious to see how much imagery I should add.
This is a difficult question to answer. In my mind, it depends upon your goals in writing the story. In technical terms (if one studies the craft books on writing), it depends upon the genre of the story. For example, if this romance is embedded within a thriller, you might want to reduce the level of imagery to a minimum; too much description may well get in the way of the rapid advancement of the plot. On the other hand, you might want to create a narrative masterpiece that captures in great detail the background against which the romance takes place. Gentile lovers living out a story of manners and culture. I can believe that you could find readers for both of these approaches, and many more, no doubt.
The best advice that I can give you is to envision who your ideal reader is. Once that you have done that, you have eliminated thousands if not millions of possible questions and answers. One you have done that, you can answer what would this ideal reader would want to read. Not an easy task, but one that will produce useful results.
If this is a true-love-lost story, then too much imagery (especially bloody or especially realistic and repellent sickness) will ruin the romance. It is not generally done, the dying partner is typically kissable until the end; they aren't vomiting blood, burned beyond recognition and still alive, or covered with blood and screaming from a car accident. You have to find an "acceptable" death that allows for final words, vows, and mutual tears. So on a scale of 1 (hardly noticeable) to 10 (horrific) keep in the 1-3 range; mild imagery. Knowing that death is imminent is all you really need.
If what you WANT is violent imagery, make similar imagery scattered throughout the story, especially so in the earliest scenes, so the audience is inured to it. What happens repeatedly can add tension to the love story, this is happening in a dangerous place or situation, and the audience fears from the beginning one lover or the other will meet a grisly fate. Do not spring this kind of imagery on them at the end, that will ruin the story for them.