There few, is any, records for National's production numbers. I know, I been looking for my '51 Chevy National Imperial series production numbers for a long time. If you ever find anything let me know and I'll do the same.
I do know that there are not a lot around especially ambulances. Hearses were usually taken care of due to presentation. Ambulances were workhorses. Lot more mileage, usually, and putout to pasture when done.