The Trending Collections section is based on revenue or sales volume. My suggestion is to also create a section that shows collections with the highest activity measured by number of transactions. For example: A collection with 10 sales of $10 each would be ranked higher in the highest activity section than a collection that with 1 sale of $100 in that same period. However, both collections would have equal ranking in the Trending Collections section. It is just a way for users to quickly identify collections that are gaining positive momentum in terms of activity.