$('.up-btn').on('click', function(e){
  var cnt =  $(this).siblings('input').val();
  cnt++;
  $(this).siblings('input').val(cnt);
});

$('.down-btn').on('click', function(e){
  var cnt =  $(this).siblings('input').val();
  cnt--;
  $(this).siblings('input').val(cnt);
});

<label for=""><input type="number" value="" placeholder="123"><span class="up-btn"></span><span class="down-btn"></span></label>
<label for=""><input type="number" value="" placeholder="123"><span class="up-btn"></span><span class="down-btn"></span></label>
<label for=""><input type="number" value="" placeholder="123"><span class="up-btn"></span><span class="down-btn"></span></label>