Local Cluster, Sol System. Yeah. Well, then, I can't really say. Clusters and all vary wildly in size. There are 22 of them, though, according to the wiki.
...Why? The only point to the resources is to pay for upgrades. After you've beaten most of the game, what do you need upgrades for? Well, okay, you'll probably get a bonus in Mass Effect 3 for importing a character, same as you did from ME1, but still. That is so damn boring.