The highlighted text below, from Goldman Sachs, explains it all.
Step 4: Repeat Step 1-Step 3 as many times as possible, during the period of LC (usually 6 months, with range of 3-12 months). This could be 10-30 times over the course of the 6 month LC, with the limitation being the amount of time it takes to clear the paperwork. In this way, the total notional LCs issued over a particular tonne of bonded or inbound copper over the course of a year would be 10-30 times the value of the physical copper involved, depending on the LC duration.
And in context: