https://serverfault.com/questions/340837/how-to-delete-all-but-last-n-zfs-snapshots#340846

You may find something like this a little simpler

zfs list -t snapshot -o name | grep ^tank@Auto | tac | tail -n +16 | xargs -n 1 zfs destroy -r

    output the list of snapshot (names only) with zfs list -t snaphot -o name
    filter to keep only the ones that match tank@Auto with grep ^tank@Auto
    reverse the list (previously sorted from oldest to newest) with tac
    limit output to the 16th oldest result and following with tail -n +16
    then destroy with xargs -n 1 zfs destroy -vr

deleting snapshots in reverse order is supposedly more efficient.

or sort in reverse order of creation

zfs list -t snapshot -o name -S creation | grep ^tank@Auto | tail -n +16 | xargs -n 1 zfs destroy -vr

Test it with ...|xargs -n 1 echo