How many ads am I able to put on the side of a page?

I have a couple posts that run a bit long and thus create a lot of white space on the side because I only have a couple ads. For instance, one this post, once you scroll down far enough there seems to be too much white space. http://www.todd-william.com/anecdotal-issues/ AM I able to just fill this with more ads? Or, what would you recommend filling this with (or any other solutions). Thanks for any advice.