Basically you can create as many ads as you like, however you can only use three at a time at most on each page.
Personally I create quite a few different ads and edit colour, style etc according to the web site where I am placing them.
When it comes to placing the ads, my best performers are the 120x600 tower and the top banner, while I also place a 250x250 box above the fold if possible.
You can see an example of my ad placement here http://www.australianstockexchangeshareprices.com.
It really is a question of split testing until you see what colours, placements, ad types work best for your site.
Hope that helps,