List the state in the US with the most invoices.
SELECT billing_state ,  COUNT(*) FROM invoices WHERE billing_country  =  "USA" GROUP BY billing_state ORDER BY COUNT(*) DESC LIMIT 1;
SELECT billing_state ,  COUNT(*) FROM invoices WHERE billing_country  =  "USA" GROUP BY billing_state ORDER BY COUNT(*) DESC LIMIT 1;
[{'billing_state': 'CA', 'COUNT(*)': 21}]