One of the great things about eMiniMalls is that there are no restrictions on how many ad units you can place on a site. I would strongly advise that you do not place too many on a page - but that you consider at least two and maybe even three depending upon the design and length of your pages.
One strategy that I’m seeing many site owners take up is to place three ads per page in the following way:
Ad 1 - this is a 468 x 60 pixel ad unit that appears at the top of articles (either just above or just below the article title). This ad would be targeted upon the title as it’s keyword
Ad 2 - this ad is often a 468 x 180 pixel ad unit that appears at the bottom of an article - just above comments. Depending upon the length of your average article - this post could also be targeting the post’s title - or if you want more variety it could target and rotate some more generic product keywords that your site is about.
Ad 3 - this ad is often one of the narrow and often small ads (maybe a 160 x 160 or a 120 x 600 depending upon available space) and appears in a side bar. These ads are usually targeting and rotating keywords that are relevant to the overall site’s theme.
For more on Ad positioning you might like to read a short series of posts that I’ve previously written on the topic of Adsense Ad Positioning that mentions some of these principles.