Hello Balázs,

Thanks for reaching out to us.

Let me forward this question to the developers so we can get a precise answer. I will report back as soon as i have feedback from them

Thank you for your patience and understanding.

Hello Balázs,

Depending on the hook you use, it can be achieved with the hook that has $container as a parameter. For example:

function my_custom_function ($reservationData, $container) {
    if ($container && $reservationData && isset($reservationData['booking']['customerId'])) {
        $customers = $container['domain.users.customers.repository']->getFiltered(['customers' => [$reservationData['booking']['customerId']]]);
        $customerTotalAppointments = $customers[$reservationData['booking']['customerId']]['totalAppointments'];
pa    }
}
add_action( 'AmeliaBookingAddedBeforeNotify', 'my_custom_function', 10, 2 );

Let us know if it works.