Entry count filter to count field values

custom.php

<?php

add_filter('quform_entry_count_1', function () {
    return my_get_field_value_count('1_15');
});

add_filter('quform_element_valid_1_15', function ($valid, $value, $element) {
    $form = $element->getForm();
    $currentCount = my_get_field_value_count($element->getIdentifier());
    $remaining = $form->config('entryLimit') - $currentCount;

    if ((int) $value > $remaining) {
        $element->addError("Sorry, there are only $remaining passes left");
        $valid = false;
    }

    return $valid;
}, 10, 3);

function my_get_field_value_count($elementId) {
    global $wpdb;
    $repository = quform('repository');

    list($formId, $elementId) = explode('_', $elementId);

    $values = $wpdb->get_col(
        $wpdb->prepare(
            "SELECT value FROM " . $repository->getEntryDataTableName() . "
            LEFT JOIN " . $repository->getEntriesTableName() . " ON entry_id = id
            WHERE form_id = %d AND element_id = %d",
            $formId,
            $elementId
        )
    );

    $count = 0;

    if (is_array($values)) {
        foreach ($values as $value) {
            $count += (int) $value;
        }
    }

    return $count;
}

Replace the number 1 on line 3 with the form ID.
On lines 4 and 7, replace 1_15 with the unique ID of the "How many passes?" field.
If you are copy & pasting this into another PHP file or plugin, do not copy line 1 or it may cause an error