JSFiddle - React, Tailwind, and code Playground

HTML

<p>Количество:</p>

<div class="number">
  <input type="text" value="1">
</div>

<p>Цена:</p>
<div class="price"><span>2 500</span>руб.</div>

JavaScript

(function () {
   var inputs = document.querySelectorAll('div.number>input'),
     len = inputs.length,
     data = '<span class="plus" onclick="addto(this);">+</span>\
            <span class="minus" onclick="addto(this,1);">-</span>';
     for (; len--;) inputs[len].insertAdjacentHTML('afterend', data);
     addto = function (el, val) {
     var par = el.parentNode || el.parentElement,
     inp = par.children[0],
     inpvalue = +inp.value;
     inp.value = inpvalue + (val ? inpvalue > 1 ? -1 : 0 : 1);
      }
}());