Will the more sites you add to your AdSense account mean the less Google will pay for each impression/click? Also if i made a website with many features and a forum was one of those features would google pay me less due to having a forum on the site?
This is actually a bit more complex than it might first seem. Having lots of sites won't directly cause your earnings to drop. However one of your sites might convert extremely badly... that could get you smartpriced causing less earnings over your total network. As far as I know smartpricing affects your whole account and is not limited to the site that converts badly.