I think he might be referring to the individual panels, and not the entire sliding door. In any case, I think there might be some slight confusion because the whole door is usually covered by one sheet of shoji paper. The individual panels aren't papered one-by-one - except in the case where a panel might have been repaired. When a kid pokes a hole in a panel, as is common even in Japan, people either carefully cut out the paper for that specific panel and replace it with new paper. Maybe there will be some leftover material from when the shoji was originally papered, or you can buy "repair shoji paper" at any hardware store. They also sell little band-aid type patches of shoji paper for you to place over finger holes and other slight damages. You often see them in the shape of cherry-blossoms..
In places where shoji are habitually poked and ripped (say, restaurants or ryokan or houses with toddlers) it is very common to see shoji that has been repaired a million times. If you don't like the look of the repaired shoji, you can replace the paper for the whole door in one shot, but it gets to be a very tedious job, which is why houses with kids learn to live:
a) with holes in their shoji
b) with patches in their shoji
c) or if you are obsessive about this, you get to be very good at replacing shoji paper