@LucieTheLakie, when my D (HS grad '15) got her merit letter it said that 2% of applicants (so about 600 if you figure 30,000 applicants) were awarded full tuition. Of course that includes OOS students.
It does seem to depend on major within Pitt. Engineering instate students even with very high stats reported that they got $10,000 or $5,000 merit. When OOS students reported receiving full tuition with similar stats, but of course they also pay about $10k more for OOS tuition.
My D applied to Arts and Sciences. So not sure if it’s easier to get merit there.