There’s a 2012 Education Week blog post by Rick Hess that explains that Barron’s uses four factors in determining degree of competitiveness: high school class rank, high school grades, standardized test scores and the school’s selectivity rate (presumably acceptance rate). Barron’s hasn’t changed the cut-offs between levels in years, with the result that more and more schools qualify as most competitive as GPA’s have risen and acceptance rates have fallen. Hess notes that in 1991, 44 schools ranked as most competitive but by 2011, 87 did. It’s not that more schools are being rated, it’s that the distribution among the categories is changing.
I applied to college in 1980. To the best of my recollection, there were about two dozen schools in the most competitive category.