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  $orders = Bitrix\Sale\Order::getList([
                                        'select' => ['*','CNT']
                                        ,'filter' => ['USER_ID' => $USER->GetID()]
                                        ,'order' => ['ID'=>'DESC']
                                        ,'limit' => $nPageSize
                                        ,'offset' => $nPageSize*($nPage-1)
                                        ,'runtime' => array(
                                                    new Entity\ExpressionField('CNT', 'COUNT(*)')
                                                )
        ]);                        

        $arItems = [];                            
        foreach ($orders as $orderItem ) {
            $orderItems = $orderItem;
            $order = Sale\Order::load($orderItem['ID']);
            $basket = $order->getBasket(); /// потому что orderItem - это массив!
        ...
     }

$orders = Bitrix\Sale\Order::getList([
    'select' => ['*','CNT']
    ,'filter' => ['USER_ID' => $USER->GetID()]
    ,'order' => ['ID'=>'DESC']
    ,'limit' => 21
    ,'offset' => 10
    ,'runtime' => array(
                new Entity\ExpressionField('CNT', 'COUNT(*)')
            )
]);

var_dump($orders->getSelectedRowsCount());