In my forthcoming book on early U.S. Indian relations, I observe that in 1793, southern Indian warriors seeking to drive American settlers out of the Tennessee Valley were able, with the use of multitribal meetings and the promise of Spanish supplies, to assemble a large force of gunmen for an assault on Knoxville. This Indian army comprised nearly 2,000 men from the Creek, Cherokee, and Shawnee nations, and was one of the largest Native American military forces to operate in a single campaign in the eighteenth-century southeast.
Apparently, though, this was only a large force for its day, and would not have been considered so two centuries earlier. In his detailed and archaeologically-informed history of the De Soto expedition of 1539-43, Charles Hudson gives this description of the Spanish adventurers' first encounter with Indian travelers on the Mississippi River:
"Some Indians in dugout canoes approached and pulled up to the landing. Four principal men got out and approached De Soto...They informed De Soto that they were vassals of a great chief, Aquijo, who was dominant over many towns on the opposite side of the river. They said Chief Aquijo would come the next day to talk with De Soto. The next day the chief arrived with a fleet of two hundred very large dugout canoes, full of Indians armed with bows and arrows. The Spaniards counted as many as seven thousand Indians, painted with red ocher and wearing feathers of many colors. Some of them held shields of cane so tightly and strongly woven that a crossbow bolt could hardly penetrate them." (Knights of Spain, Warriors of the Sun [University of Georgia Press, 1997], 284-5.)
That was just the warrior population of one chiefdom in Arkansas.
I know of no more vivid way to illustrate the decline of Indian population in the southeast between 1540 and 1790. Moreover, white settlers and the U.S. government had considerable difficulty defeating and expelling the remnant Indian nations they encountered in the southeast in the 18th century. If they had instead encountered Indian chiefdoms the size of Aquijo's, I suspect it would have taken Americans the better part of a century to conquer them. (And if those chiefdoms' warriors had been armed with muskets and cannons, I suspect we would never have heard of "Manifest Destiny.")