I also don't imagine huge international transactions in gold or silver. We only have about 160,000 tonnes of gold in the world, total - 0.024kg or less than one ounce of gold for each of the 6.7 billion people on Earth. Or if we consider silver, about 43 billion ounces have been produced ever, about 6 ounces each. The medieval silver penny had a bit over a gram of silver, so that 6 ounces would allow some 170 pennies.

Would this really be enough currency for all the transactions people want to make?

My point was not that any particular way of having currency was the best, but that there were very many different ways of doing things throughout history - not just silver and gold.