I wonder how many manuscripts are submitted daily to the top scientific journals, such as Nature or Science. Each of those journals has multiple editors, specialising in narrower fields. The question that follows is how many submittions does one editor handle daily?
According to Nature's website, they received 10,952 submissions last year, which works out to an average of almost exactly 30 papers per calendar day. Given that there are 26 subject area editors according to their editorial masthead, it would appear that the average editor gets about 8 papers per week, or one or two per day.
However, their caseload is probably much bigger, since they also have to deal with all of their other "open" papers as well—so an editor might have something like 60-100 active papers to deal with at any one time (ballpark estimate, assuming a two- to three-month life cycle—although I suspect it's actually much longer than that if multiple rounds of reviews are required).