Two a year with two being mothballed at the 12-15 year mark. Keep them until the 24-30 mark and sell off the half used ships to friendly nations.
In event of war we can have a 48-60 ship fleet of fairly modern well taken care of ships available plus facilities capable of making them at a moments notice.
In peace we have spares and options as well as stable jobs and realistically cost effective manufacturing due to constant known demand/supply.