JSFiddle - React, Tailwind, and code Playground

HTML

<input id="testInput" onkeyup="limitWords(this.id)" />

JavaScript

function limitWords(id) {
    var maxWords = 3;
    var d = document.getElementById(id);
    if (d.value.split(' ').length > maxWords) {
        t = d.value.substring(0, d.value.lastIndexOf(' '));
        d.value = t.substring(0, t.lastIndexOf(' ') + 1);

        // Ensure title case
        d.value = toTitleCase(d.value);

        alert("You can choose up to 3 sectors");
    }
}

function toTitleCase(str) {
    return str.replace(/\w\S*/g, function(txt) {
        return txt.charAt(0).toUpperCase() + txt.substr(1).toLowerCase();
    });
}