More than 6 Keys in a wad?

Up to 255 new keys can be used.

You have to create new DECORATEs for each new key.

Then define a LOCKDEFS for the key.

A while back I made a tutorial describing a method to use a green keycard to open a door with a specific message. 

More precisely: you can define up to 255 locks. However, you don't necessarily have a 1:1 relationship between locks and keys. And you can always use different methods to add what is functionally more locks.

I wrote about that on the ZDoom forums.