How many transactions correspond to each invoice number?
SELECT invoice_number ,  count(*) FROM Financial_transactions GROUP BY invoice_number
SELECT invoice_number ,  count(*) FROM Financial_transactions GROUP BY invoice_number
[{'invoice_number': 1, 'count(*)': 2}, {'invoice_number': 2, 'count(*)': 1}, {'invoice_number': 3, 'count(*)': 1}, {'invoice_number': 5, 'count(*)': 1}, {'invoice_number': 6, 'count(*)': 1}, {'invoice_number': 12, 'count(*)': 4}, {'invoice_number': 13, 'count(*)': 1}, {'invoice_number': 14, 'count(*)': 2}, {'invoice_number': 15, 'count(*)': 2}]