Sales Order – Filter Orders by Customer Email ID

Error in column code use below code.

$this->addColumn('sales_rep', array(
    'header' => Mage::helper('sales')->__('Sales Rep'),
    'index' => 'sales_rep',
    'type'  => 'text',
    'width' => '70px'
));

you have used 'type' => 'options' and not define options there

Please try this:

$orders = Mage::getModel('sales/order')->getCollection()
    ->addAttributeToFilter('created_at', array('from' => $fromDate, 'to' => $toDate));
$array = array_count_values($orders->getColumnValues('customer_id'));

foreach ($array as $customerId => $orderCount) {
    echo "Customer ID : " . $customerId . " Order Count : ". $orderCount. "<br>";
}

Explanation:

This stores customer_id for all orders in an array ...

$orders->getColumnValues('customer_id')

... and array_count_values sums same array values and stores it as array again.

Edit:

If you have orders, where customer_id is null then array_count_values gives an error (Warning: array_count_values(): Can only count STRING and INTEGER values!).

So you have to replace null first:

$array = array_count_values(array_map(function($v) {
    return (is_null($v)) ? "?" : $v;
}, $orders->getColumnValues('customer_id')));