Once The Midwest California Superliner order is filled, how many single level "corridor" cab cars will be needed in general national wide service? My answer would be None! Any existing NPTU cabbage units would suffice.
If I'm understanding your comment correctly - moving the existing NPCUs built from F40PH units to corridor trains - that will not work due to the clearance in the tunnels in and out of New York Penn Station.
As well, the impression I have been getting is that the former F40PH NPCUs are difficult to keep in good order, and I do know firsthand that they are very uncomfortable to ride, with a lot of hard side-to-side action on switches, diamonds, etc. I'm of the opinion that they have served a purpose for a while, but it's time to look to phase them out with new car orders.