Yes, but you can add as many new stamps as you want. It will also blend them together, so you might see some things that look kind of familiar but when you're on the ground it doesn't look at all the same and I've ran many many many seeds. Also, modders can add to the terrain stamps using png files, there is a base height level (r:32 g:32 b:32) and anything darker will dig down, anything higher will build up. Once I go over how to use our r16 format you'll even be able to define lakes at certain heights IN a stamp. It's really nifty. a20 (gold) RWG has a LOT more content driven design than any iteration before it. I'm very confident people will be happy with how it generates and modders will be thrilled at having a fairly easy way to build customized random maps. (looking at you, total conversion modders ;))
Anyway, I'm trying to avoid talking about the details too much on here. I'm super excited to show off / talk about everything and have a whole list of stuff that I'll be going over on the dev stream on October 13th (this week is the other programmers).