I go to a top 25 school and I would say of the ~10-12 people applying into ophtho this year (myself being one of them), one student had a first author poster, one had a 5th author poster and one had 1 poster as first author and gave a talk as a third year. So 25-33% with at least one poster and 10% with more than one.
I really don't know what it's like at other med schools, so I'll refrain from trying to give a percentage....