User-selectable Limit

&limit=`[[!++results_to_show]]`

if (isset($_POST['results_to_show']) && isset($_POST['submitValue'])) {
    $userId = $modx->user->get('id');
    $key = 'results_to_show';
    $value = $_POST['results_to_show'];

    $setting = $modx->getObject('modUserSetting', array('user' => $userId, 'key' => $key));

    if (! $setting) {
        $setting = $modx->newObject('modUserSetting');
        $setting->set('user', $userId);
        $setting->set('key', 'results_to_show');
    }

    $setting->set('value', $value);
    $setting->save();
}
return;

<form method="post">
    <select>
        <option value="10">10</option>
        <option value="20">20</option>
        <option value="0">View All</option>
    </select>
</form>

($_POST['results_to_show'])

<?php
$value = 10;

if (array_key_exists('results_to_show', $_POST) && is_numeric($_POST['results_to_show'])) {
  $value = (int)$_POST['results_to_show'];
}

// make it available as [[!+results_to_show]]
$this->setPlaceholder('results_to_show', $value);
// OR
// if you want to use the snippet inside the call directly
return $value;