JSFiddle - React, Tailwind, and code Playground

HTML

<div class="number">0</div>

CSS

body {
    height: 2000px;
}

.number {
    position: fixed;
}

JavaScript

var body = document.body,
		html = document.documentElement,
    number = document.querySelector('.number'),
    max = 300;

window.addEventListener('scroll', function() {
		var scrollHeight = (html.scrollHeight || body.scrollHeight) - this.innerHeight,
        scrollTop = html.scrollTop || body.scrollTop || 0;
		number.innerHTML = Math.round(max / scrollHeight * scrollTop);
});