Because the new coins issued in ancient Rome were worth less than the older coins merchants demanded more new coins for the same product what is the principle called?

The principle you're referring to is known as Gresham's Law, which states that "bad money drives out good." In this context, the newer coins with less intrinsic value (bad money) would circulate more widely as merchants preferred to keep the older, more valuable coins (good money) for themselves, leading to an increase in the quantity of new coins required for transactions.