Once the user completes 5 purchases, the code automatically applies a discount to their 6th purchase. After applying the discount, the order count resets.

Discount 6th order woocommerce.php

add_action('woocommerce_order_status_completed', 'track_completed_orders_101', 10, 1);
function track_completed_orders_101($order_id) {
    $order = wc_get_order($order_id);
    $user_id = $order->get_user_id();

    if ($user_id) {
        $completed_orders = get_user_meta($user_id, '_completed_orders_count', true);
        $completed_orders = $completed_orders ? $completed_orders + 1 : 1;
        update_user_meta($user_id, '_completed_orders_count', $completed_orders);
    }
}

add_action('woocommerce_cart_calculate_fees', 'apply_sixth_purchase_discount_101');
function apply_sixth_purchase_discount_101() {
    if (is_admin() && !defined('DOING_AJAX')) return;

    $user_id = get_current_user_id();
    if ($user_id) {
        $completed_orders = get_user_meta($user_id, '_completed_orders_count', true);

        if ($completed_orders >= 5) {
            $discount_percentage = 10; // Set the discount percentage here
            $discount = WC()->cart->get_subtotal() * ($discount_percentage / 100);

            WC()->cart->add_fee('Loyalty Discount', -$discount);
        }
    }
}

add_action('woocommerce_order_status_completed', 'reset_order_count_after_discount_101', 20, 1);
function reset_order_count_after_discount_101($order_id) {
    $order = wc_get_order($order_id);
    $user_id = $order->get_user_id();

    if ($user_id) {
        $completed_orders = get_user_meta($user_id, '_completed_orders_count', true);

        if ($completed_orders == 6) {
            update_user_meta($user_id, '_completed_orders_count', 0);
        }
    }
}