What percentage of admitted phd students have peer-reviewed publications?
I don't have statistics on that particular number, but my anecdotal evidence is that most PhD students in my field (chemical engineering) have zero publication when they start, while a small (but nonnegligible) fraction has one paper.

However, you can gather such statistics yourself! Publications are, by definition, public... And you can sample admitted PhD students by looking at current PhD students. So, take a random sample of CV from PhD students in your field (from the web) and look for publications prior to PhD start in their publication list.