[Mike23]

var elem = document.querySelector('.elem');
 var buttonPlus = document.querySelector('.button_plus');
 var buttonMinus = document.querySelector('.button_minus');
 var num = 0, max = 5, min = 0;
 //var num = +elem.innerHTML;

 buttonPlus.addEventListener('click', addElem);
 buttonMinus.addEventListener('click', removeElem);

 function addElem() {
 num = Math.min(++num, max)
 elem.innerHTML = num;
 }

 function removeElem() {
 num = Math.max(--num, min)
 elem.innerHTML = num;
}

Спасибо большое!