JSFiddle - React, Tailwind, and code Playground

HTML

<span id="s1">Lorem Ipsum</span>
<span id="s23">Lorem Ipsum</span>
<span id="s6">Lorem Ipsum</span>

JavaScript

var spans = document.getElementsByTagName("span");
var length = spans.length;
var highest = 0;
for(var i = 0; i < length; i++) {
    var id= parseInt(spans[i].id.substring(1, spans[i].id.length), 10);
    if(id > highest) {
        highest = id;
    }
}
alert(highest);