I voted "any number as long as their consistent across resolutions."
The reason for this is simple. Having a "Vanilla" version of DC to build with would be a really great thing to get people building right out of the gate without having to fumble around looking for art assets. Some users might prefer to have all that sort of stuff already in the toolset, working as it should, without any additional work on their part. And a lot of RPG toolsets are like that anyway.
Some people might find it daunting to have to seek out and figure out how to get the walls they want versus having a whole lot of them already available and ready to go. Not everyone is like the people on this forum.
I think the same could be said for backdrops and other art assets as well. Give a base version with a lot of art that people can work with if they want to. Those who are more experienced will find it trivial to change everything to suit their needs.
By the way, if you're looking for a pool of small pics, feel free to use any of the ones I've posted here to include in DC (some are 90% your work anyway, so you can make the calls on that).