JSFiddle - React, Tailwind, and code Playground

HTML

<textarea name="ta1" id="ta1" rows="4" cols="50">
 ABC
 DEF
 GHI
 </textarea>
 <input type="text" name="textnumber" value="">

CSS

#ta1 {
  display: block;
  margin-bottom: 30px;
}

JavaScript

var $textarea = document.getElementById('ta1');
var $input = document.getElementsByTagName('input')[0];

var newlines = ($textarea.value.match(/\n/g)||[]).length;
$input.value = newlines;