It’s difficult to answer the question without knowing your particular layout, the products on the trolleys, the trolleys’ orientation and other factors that might impact read rates. But I would think that placing tags on the four sides of each trolley should guarantee their ability to read every tag every time. This would be a much cheaper solution than purchasing additional interrogators.
I would also suggest you consider the types of tags you buy, and their placement, carefully. Confidex makes a tag that hangs off a trolley, keeping it away from metal, thereby enabling the tag to be read consistently. Container Centralen has used this hanging tag, and I believe it requires only one per trolley (see Container Centralen Says It’s Ready to Roll Out RFID in Europe).
—Mark Roberti, Editor, RFID Journal