Wikipedia: It is difficult to generalize what percentage of top marks correspond to each of the degrees of honors, given that the percentages or grade point averages required for each rank can differ from place to place or university to university. For example, at Harvard, between 4% and 5% of the graduating class receive summa, while 20% receive either summa or magna.[2] In general, degrees summa cum laude are relatively rare—often reserved for the top two percent of students at the most—and degrees magna cum laude only slightly less so.[citation needed]
The wikipedia article is referring to Harvard College. HLS has specific criteria (10% magna, maybe 30% cum laude; summas virtually impossible — one every few years] which are findable through their websites. [This refers to the time prior to their recent near-abolition of grades, aboout 2 years ago]