You can place as many as you want as long as each ad unit displays a different item. I currently have 5 ads running on the frontpage of my site in a row. This is where choosing your own keywords for the ads comes in really handy.
The Chitika blog says that there's no limit but they recommend no more than three. If you want more than that it's best to mix Chitika with Adsense.
I agree completely. You don't want to put all your eggs into one basket either. I have found that having non-contextual Chitka on my frontpage and Google Adsense on my classified listing pages works excellent. I also change the keywords on the Chitika ads every couple of days to showcase a new product.