Theory:
The more items we have, the more eager we are to complete the series.
Application:
People like to collect and complete the set. Incomplete puzzles or images are a nag to our brain. Can you divide your product range into several (logical) groupings that people can 'collect'? Or you can show them together to suggest the items being in a set to stimulate people to buy them all. You can show sets on your webshop, or e-mail people that already bought item A an B that they really should buy item C to complete the set!
Recently I've seen some (often absolute) statements going around, generally in the line of "open source commerce platforms are a terrible idea". Now of course different solutions always have different pros and cons.
A hierarchy of evidence (or levels of evidence) is a heuristic used to rank the relative strength of results obtained from scientific research. I've created a version of this chart/pyramid applied to CRO which you can see below. It contains the options we have as optimizers and tools and methods we often use to gather data.
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