The number of physical maps can't be changed. You can "clone" any existing land mass into a new facet as many times as you wish inside MapDefinitions.cs. You can also replace any of the existing maps with a new one, either created yourself or downloaded/purchased elsewhere. I'm sure that part is covered in detail somewhere on the site here.
I've used a purchased Darkshard map to replace Fel while keeping the original Tram, something that can't be done on the latest clients. Replacing the other maps is easier. I did have to tweak the housing placement code because the new map has a "rougher" terrain that would prohibit housing in most spots.