From asking during my externships about number of externs this year vs others, it seems that the number of externs might be down some this year, at least where I did externships....Probably doesn't mean much 😉
Its probably tough to tell how many total applicants in the pool - not all applicants apply to any one program, so each program effectively sees a different subset of the applicant population (some see more or less). You do have last year's matching outcomes on the NRMP site, so thats probably a decent guide.