The short answer is that there were times in which people in North America (and here I assume you mean north of Mexico, because once we get into Mesoamerica this becomes all the more obvious) did coalesce into large political entities. Today there are 566 Federally Recognized tribes in the United States, and 614 First Nation governments in Canada (plus, in the US at least, there are also 60+ State recognized tribes, and a couple hundred unrecognized tribes with varying degrees of legitimacy). One of the reasons these numbers are as high as they are is due to the way the former larger polities have been fractured in recent history.

There's a considerable amount of ambiguity in the word tribe. Part of the reason there are so many "tribes" is because the word has been used to refer to an indigenous society of just about any size. Even then, an estimate in the millions would be absurdly high for North America. In American anthropology, we try to be a bit more restrictive with the term, but even in academic circles it can be hazy. I prefer to use more specific terms when I can, or the deliberately generic "polity" when I can't. So with that in mind, let's look at a few historic Native polities as examples.

• The Haudenosaunee: More famously known simply as the Iroquois, The Haudenosaunee formed in what's now upstate New York as a confederacy of five nations (Seneca, Cayuga, Onondaga, Onedia, and Mohawk) either in the mid-12th Century or sometime before the mid-15th Century (depending on whether you side with the early-daters or late-daters). In 1722, the Tuscarora were added as a sort of protectorate initially and elevated to full membership after the American Revolution. So does this count as one "tribe" (the Haudenosaunee / Iroquois Confederacy), as 6 tribes (the individual member nations), or more than a dozen (just counting the major recognized Tribes and First Nations in the US and Canada that descend from the original Haudenosaunee). In any case, for the Haudenosaunee, we do see the coalescence of a larger political entity in precolonial times that eventually fractures during the late colonial period, particularly during the American Revolution.

• The Oceti Sakowin: This is another major precolonial confedearcy, literally the Seven Fires Council but more often called the Great Sioux Nation in English. We know less about how this one operated in its prime than we do for the Haudenosaunee because by the time they started interacting with Europeans the confederacy was already becoming a victim of its own success. Originally based in what's now Wisconsin and Minnesota, the confederacy consisted of the Mdewakanton, Wahpeton, Wahpekute, Sisseton (collectively the Dakota), the Yankton, the Yanktonai (collectively the Western Dakota, or sometimes erroneously as the Nakota), and the Titunwan (the Lakota). Each of these had further subdivisions - like the Hunkpapa and Oglala among the Titunwan, many of which are their own federally recognized tribes today. As the Oceti Sakowin spread westerward during the colonial period with the Lakota leading the charge, the confederacy was increasingly difficult to hold together. The Lakota and Dakota had fewer and fewer common interests going into the 1800s, and the Oceti Sakowin became less of a practical political concern and more of a symbolic cultural one. So again, which level do we properly call a tribe: the Oceti Sakowin, the Dakota and the Lakota, the Sisseton, Yankton, Titunwan, etc., or the Oglala, Hunkpapa, etc.?

• Coosa: You might not have heard of Coosa before. It's not as well know as the previous two examples, so this previous post will give you a more detailed overview than I'll go into here. At the time of European contact, it was a polity that stretched for nearly 400 miles along the southern Appalachians, with its capital, Coosa itself, in northwest Georgia. It came through its initial contact with Europeans in the early 1540s relatively unscathed, but it seems that people severely lacked confidence in the leadership of the micco, viewing him as capitulating too easily to the Spanish and failing to protect them of excessive demands. Coosa crumbled as a major independent polity, but regained some of its prestige by becoming one of the four Mother Towns of the Creek Confederacy which formed after the collapse of the early colonial era Muskogean Mississippian powers. The Creek Confederacy held sizable land claims in what's now Georgia, Alabama, and northern Florida. This map shows their territory in Georgia and Alabama; the Creeks in Florida eventually split off from Confederacy to become the Seminoles before major land cessions began. The political forces at work driving the land cessions depicted in that map also further fractured the Creek Confederacy. Most were forced to relocate to Oklahoma becoming the modern Muscogee (Creek) Nation along with three independent smaller Tribal Towns; others managed to stay, becoming the Poarch Band of Creek Indians today (all federally recognized groups). There are also some State recognized offshoots of the Creek Confederacy in Alabama and Georgia. So again we see Native peoples coalescing into larger polities, fracturing, coalescing again, fracturing again, resulting in many modern tribal entities.