So far, I've found references claiming numbers from 95% to 98.8%, but for the density of medium spiny neurons in the neostriatum of MICE or RATS. I'm not sure that numbers applicable to rodents would necessarily apply to humans. Certainly, our brains are much more complicated!
After about 2 hours of searching, I've found references for "over 95% in rats and mice, and 75-80% in primates" (in a textbook, published 2007) and "90–95% in rats and over 85% in humans" in a paper published in 2008. But nowhere have I seen a value of over 90% for humans, let alone 96%.
So I've posted on the relevant Wikipedia Talk pages asking people with more time than me to look over the numbers. I don't have time to try editing Wikipedia myself, not when it's more than a very minor edit and I don't know all the codes, and I have a literature report to write. I think this will be my new thing to do whenever I find mistakes on Wikipedia and don't have time to correct them myself. It's not as useful as making the correction, but at least it points people who have time to the data.