add_filter( 'woocommerce_order_item_quantity', 'woocases_update_inventory', 10, 2 );

function woocases_update_inventory( $this, $item ) {

    $order = new WC_Order( $item->id );

    $items = $order->get_items();

    foreach( $items as $item ) {

        $case_count = null;

        if( $item['variation_id'] ) {
            $case_count = get_post_meta( $item['variation_id'], '_number_case', true );
        } else {
            $case_count = null;
        }

    }

    if( $case_count >= 1 ) {
        return $case_count * $item['qty'];
    } else {
        return $item['qty'];
    }

}