#!/bin/bash
file="${1}"
stag="${2}"
max=0
open=0
grep -Po "</?${stag}" "${file}" | while read tag; do
  if [[ "$tag" == "<${stag}" ]] ; then
    (( open++ ))
  else
    (( open--))
  fi

  echo "$open - $max"

  if [[ $open -gt $max ]] ; then
    max=$open
  fi
done