how to display running count of records in query
You'll want to use a window function such as ROW_NUMBER() for this one like so:
SELECT CUSTOMER_ID, TRANSACTION_ID, ROW_NUMBER() OVER (PARTITION BY CUSTOMER_ID ORDER BY TRANSACTION_ID) AS [Count] -- Running incremental count of rows / TRANSACTION_ID per CUSTOMER_ID
FROM TRANSACTIONS 
GROUP BY CUSTOMER_ID, TRANSACTION_ID 
ORDER BY CUSTOMER_ID, TRANSACTION_ID