This website:
http://www.japparts.nl/index.php?main_page=product_info&cPath=79_90_107_572&products_id=706
says that 1FN serial numbers are
YAMAHA FZ750(1FN) Framecode 1FN, 1TV Year 85-86 Yearcode Frame Numbers 1FN-000101-1FN-013101
which would suggest that 13000 1FN have been produced