JSFiddle - React, Tailwind, and code Playground

HTML

<p class="order_text1">Каробачки: <br> <span id="packs">39</span> packs.</p>

JavaScript

function rand(min, max){
    return (max-min)*Math.random()+min
}

var container = document.getElementById("packs"),
    from = 39,
    to = 4,
    timer = setInterval(function () {
        from === to, container.innerHTML = from--
    }, rand(1000, 4000));