Hi,
You could get a listing of all the customers with their number of orders with a report with the display mode "listing" on "customers" with the "ordering" setting set to "orders". However, the first customers of the listing will be the ones with the most sales.
And if you have a lot of customers, you might not be able to display them all to export the CSV of all the data so that you can then open the CSV in Excel to remove the rows with a count greater than one.
A simple alternative would be to just run a MySQL query in your PHPMyAdmin to get the data.
Something like that:
SELECT u.user_email,count(o.order_id) AS number_of_orders FROM #__hikashop_user AS u LEFT JOIN #__hikashop_order AS o ON o.order_user_id = u.user_id WHERE o.order_status IN ('confirmed','shipped') GROUP BY u.user_id ORDER BY number_of_orders ASC
where #__ needs to be replaced by the prefix of tables available in your Joomla configuration.