Total US population is 308,745,538.
FYI
5,100,000 American Indian/Alaska Native + one or more other race
2,550,000 American Indian/Alaska Native race alone
There are 566 Federally recognized tribes. Using Census data figures above for those who identify themselves as American Indian/Alaska Native.
Top ten tribes in the US according to **infoplease dot com have populations ranging from 23,583 to 308,013 and those top ten total 1,169,548 meaning 556^Edit correction tribes are much closer to the 4,595 figure (2.1 million). (9,189 for 5.1 million) Keep in mind these numbers represent those who identify themselves as American Indian/Alaskan Native. Those actually enrolled are probably smaller than that. There, the hard numbers.
It has been estimated that American Indians will disappear as federally recognized tribes within two generations if the current enrollment requirements are kept in force as is. We will always be here culturally as whatever tribe we belong to as long as there are people. It is our sovereign status we are fighting to protect and keep, something we have always fought to protect.
The loss of even one person eligible for enrollment brings the end of Native Americans federal status that much closer. The BIA website has an FAQ page which you might find interesting. It explains a bit more the complexity of life as a Native American who is also a US citizen, something which even many Native Americans don't fully understand.
FYI
***10 most populous cities
New York City pop. 8.245 million
Los Angelos pop. 3.82 million
Chicago 2.707 million
Houston pop. 2.145 million
Philadelphia pop. 1.536 million
Phoenix pop. 1.469 million
San Antonio pop. 1.36 million
San Diego pop. 1.326 million
Dallas pop. 1.223 million
San Jose pop. 967,487 (24 million people in these cities)
Native American/Alaska Native pop. 2.5 million
Two million people out of 308 million isn't very much, is it?
*As of the 2011 American Community Survey, the nation's population of American Indians and Alaska Natives, including those of more than one race. They made up 1.6 percent of the total population. Of this total, about half were American Indian and Alaska Native only, and about half were American Indian and Alaska Native in combination with one or more other races.
Source: 2011 American Community Survey
** I used these figures as I had a hard time loading the census site.
***I searched for the ten most populous cities and got the population figure from that city's website that showed on the search page
All figures and links current as of 24 Jul 2013.