Can you count the number of unique part fault ids that belong to each unique fault log entry id, and then add those values to the table?
SELECT fault_log_entry_id, count ( * )  FROM Fault_Log_Parts GROUP BY fault_log_entry_id
count the number of unique part fault ids that belong to each unique fault log entry id refers to COUNT(*) GROUP BY fault_log_entry_id; add those values to the table is implied in the SELECT statement to retrieve and display this information, but not literally adding to an existing table.