JSFiddle - React, Tailwind, and code Playground

HTML

<span class="data-center" data-category="group1" data-amount="1"></span>
<span class="data-center" data-category="group2" data-amount="1"></span>
<span class="data-center" data-category="group2" data-amount="2"></span>
<span class="data-center" data-category="group1" data-amount="2"></span>
<span class="data-center" data-category="group2" data-amount="2"></span>
<span class="data-center" data-category="group3" data-amount="3"></span>

JavaScript

function getCategoryTotal(cat) {
    var total = 0,
        elements = document.querySelectorAll('[data-category="' + cat + '"]');
    
    Array.prototype.forEach.call(elements, function (el, i) {
        total += parseInt(el.dataset.amount, 10);
    });
    
    return total;
}

console.log(getCategoryTotal('group1')); // 3
console.log(getCategoryTotal('group2')); // 5
console.log(getCategoryTotal('group3')); // 3