I don’t know why you received a bunch of downvotes on this post; it is actually quite an interesting question. Paradoxically, contraband and smuggling was both the Achilles heel of the tightly controlled Atlantic fleet system and the life preserver that held the Spanish monarchy afloat in the New World for much of the seventeenth and eighteenth centuries. The rapid growth of the mining sector at Potosí caused a flourishing of contraband in the South Pacific and across the Southern Cone. Most of the contraband, rather than traveling legally from Potosí to Lima, traveled east to Tucumán, then on to Asunción or Buenos Aires where it could be traded for Portuguese goods like slaves, weapons, manufactured products, sugar, and rum. The flow of silver was so steady that in a way, it became sort of like an irrigation system for new cities in the Río de la Plata, leading to the establishment and growth of new towns that connected the major trading hubs.

So to your specific question regarding the amount smuggled, records on the outputs from mines in the Americas are fairly accurate actually, which means one can examine how much of the silver made it from the mine to Europe. A lot of scholars have done that to see how much silver was lost to smuggling. According to Stanley J. Stein and Barbara H. Stein in their book Silver, Trade, and War: Spain and America in the Making of Early Modern Europe, “Buenos Aires exported some wheat, hides, dried beef, and tallow and, of course, Potosí silver—estimates of smuggled silver vary between 20 to 25 percent of total Potosí output” (33). Although Potosí seems to capture the imagination of many, in reality after 1640, production at the mine gradually began to fall. It was surpassed by other mines in New Spain, making Mexico the richest and most important colony.

The nature of smuggling also changed. Since only Spanish merchants could do business in the Americas or take part in convoys, many Spanish merchants became middlemen for European traders. They provided manufactured goods to Spanish merchants who ferried the goods to the Americas. Likewise British, Dutch, French, and Portuguese traders all sailed directly to Spanish ports and loaded their goods onto Spanish ships waiting offshore in exchange for Spanish bullion or other natural products. Or they simply sold these goods to locals along some desolate and undefended coastal location. Smuggling happened all over Spanish America for centuries. According to Allan Kuethe and Kenneth Andrien in their book The Spanish Atlantic World in the EIghteenth Century: War and the Bourbon Reforms, “by the 1690s, England received 440,000 pesos annually from its illicit trade with Spanish America (mostly through Jamaica), and an additional 1.2 million pesos from supplying manufactured goods through Seville for legal trade to America. This provided enough silver to finance all commercial activities in Asia” (72). At the beginning of the 1700s, Spain was forced to give Britain the right to import slaves into Spanish ports. These traders smuggled in a lot of other goods in addition to slaves, which further sapped Spanish revenue. Kuethe and Andrien also mention studies that indicate that “the amounts of unregistered merchandise leaving Spain for the Indies fluctuated between 50 and 90 percent” (71).

Despite the devastating toll that illicit trading took on Spain’s bottom line, the monarchy still collect spectacular amounts of tax revenue from the system, and merchants in Seville, Veracruz, and other ports profited greatly from the strict flotilla system, which maintained their monopoly over trading rights. Meanwhile, contraband kept the whole system afloat by providing the goods demanded by colonists at cheaper prices and at more regular intervals. This proved especially important later in the colonial period when British naval power surpassed Spanish power, preventing regular convoys from making the transatlantic journey. By the time the Bourbons took over Spain, contraband appeared to be the most pressing problem for the new regime.

Over the next century, they worked to undermine the contraband trade. They created new viceroyalties, which could more closely monitor trading in peripheral regions like the Río de la Plata. They also began to implement more free trade opportunities, which opened various ports to individually registered ships. Additionally, they created chartered companies who were granted monopolies over certain trade routes or products. All of these changes took a long time to be implemented, but by the end of the eighteenth century, the monopolies of the great merchants had been broken. Their reforms did prove to be fairly effect remedies for contraband. Spanish tax revenue soared, which helped bring Spain back as a first rate power by the middle of the 1700s. Many expected even more changes to imperial policies in the first half of the nineteenth century, but Napoleon intervened and changed the course of Spanish America with his invasion of Spain.