The mullions have a flat back side that the glass fits against, so one large piece of glass is all that's needed.
Our cabinet doors are built without glass in the door. Type of glass is up to the customer. The back of the door is cut out to allow for glass. Glass is secured by using decorative clips or by applying a bead of clear silicon after setting glass in door.
The individual glass openings in mullion doors are called lites. Almost any arrangement of lites is possible. Mullion doors are typically made so that one piece of glass, fit in from the back, serves as glazing for all the lites in a door. However, If you are considering using mullioned doors with clear-glass inserts, keep in mind that the interior of the old cabinets will be visible. Depending on their condition, this may not be acceptable-dark or opaque glass may be called for.