CPU usage is going to depend widely depending on a variety of factors, including camera resolution and stream type, as well as how active each cameras scene is.
As I don't know that information, or what CPU type you have I can't speak too much to that, but especially if they are HD cameras it doesn't sound too surprising. Assuming they are all the same model, you can always turn them off 1 by 1 and see where the limit is. There are a variety of tweaks to the cameras configuration pages (not in Sighthound Video) that might help, such as reducing the frame rate being sent and/or the resolution.
150mb of RAM being used per camera process is a normal value. Decoding and encoding video on its own requires a fair amount of memory, plus a bit for analytics.
You can find some example systems to compare against and details about system requirements here:
www.sighthound.com/support/system-requirements