<?PHP

for ($u = 1; $u <= $_GET['amount']; $u++) {
$_SESSION['u'.$u.'_first_name'] = $_GET['u'.$u.'_first_name'];
echo 'session : '.$_SESSION['u'.$u.'_first_name'].'<br>';
}

echo '
<form action="" id="formBook">
Amount : <input id="amount" name="amount" value="'.$_GET['amount'].'"><br>
';

for ($x = 1; $x <= $_GET['amount']; $x++) {
echo '
Name '.$x.' : <input type="text" class="form-control rounded-0" id="u'.$x.'_first_name" name="u'.$x.'_first_name" value="'.$_SESSION['u'.$x.'_first_name'].'"><br>
';
}

echo '
<input class="btn btn-primary mb-3" type="submit" name="submit" value="Next" class="button"><br>
';

?>