The minimum that needs to be edited is
one Amnesia map. It won't flow into the others and each contributor has to make sure the map ends somewhere and that you cannot somehow leak into the original game.
I was thinking of Rainy Hall to be the HUB map. There is enough space for all the doors as well as a shop and some fun secret things.
The doors would say something like:
"Archive remake by Slanderous"
"Morgue remake by Peter__P"
etc.
That's a single map. You may make the map bigger, but as for multiple levels,
one is preference.
I worked on the anniversary site a bit. It has a new design and I'll add all the information needed there soon.
http://amnesiachallenge.comxa.com/
(refresh the page if it doesn't look right)