JSFiddle - React, Tailwind, and code Playground

HTML

<form action="" method="post">
       <textarea id="texto" name="texto" rows="10"></textarea><br>
       <input type="hidden" id="linhas" name="linhas" value="0">
       Linhas: <span id="contador"></span>
    </form>

JavaScript

document.getElementById('texto').onkeyup = function() {
       count = this.value.split("\n").length;
       document.getElementById('contador').innerHTML = count;
       document.getElementById('linhas').value = count;
    }