A per curiam opinion is simply an opinion of the court without attribution to a particular justice. They are often, but not always, unanimous. They are sometimes used to correct errors by the lower court where a new principle of law is not being made. Sometimes they are used to remand the case back to a lower court for reconsideration in light of a recent Supreme Court opinion.
I've never done a count, but I would think per curiam opinions affirming the lower court would be fairly unusual unless it was on entirely different grounds than the lower court used. Otherwise, the Supreme Court would normally just deny cert.