var container = document.querySelector(".mainslider");
//or
console.log(container.childElementCount);
//or
console.log(container.children.length);
calc(100% / X);
<link href="/apple-touch-icon-57x57.png" rel="apple-touch-icon" sizes="57x57">
<link href="/apple-touch-icon-60x60.png" rel="apple-touch-icon" sizes="60x60">
<link href="/apple-touch-icon-72x72.png" rel="apple-touch-icon" sizes="72x72">
<link href="/apple-touch-icon-76x76.png" rel="apple-touch-icon" sizes="76x76">
<link href="/apple-touch-icon-114x114.png" rel="apple-touch-icon" sizes="114x114">
<link href="/apple-touch-icon-120x120.png" rel="apple-touch-icon" sizes="120x120">
<link href="/apple-touch-icon-144x144.png" rel="apple-touch-icon" sizes="144x144">
<link href="/apple-touch-icon-152x152.png" rel="apple-touch-icon" sizes="152x152">
<link href="/apple-touch-icon-180x180.png" rel="apple-touch-icon" sizes="180x180">
<link type="image/png" href="/favicon-32x32.png" rel="icon" sizes="32x32">
<link type="image/png" href="/favicon-194x194.png" rel="icon" sizes="194x194">
<link type="image/png" href="/favicon-96x96.png" rel="icon" sizes="96x96">
<link type="image/png" href="/android-chrome-192x192.png" rel="icon" sizes="192x192">
<link type="image/png" href="/favicon-16x16.png" rel="icon" sizes="16x16">
<link href="/manifest.json" rel="manifest">
<link href="/safari-pinned-tab.svg" rel="mask-icon" color="#71bf44">
<meta name="apple-mobile-web-app-title" content="Textile Federation">
<meta name="application-name" content="Textile Federation">
<meta name="msapplication-TileColor" content="#ffffff">
<meta name="msapplication-TileImage" content="/mstile-144x144.png">
<meta name="theme-color" content="#ffffff">