…the vast majority of Protestants in the USA can be found under the umbrella of less than 30 denominations.
Doings some research, I think I was pretty close.
For 2004, it was estimated by
adherents.com that there were approximately 224,437,959 Christians in the United States.
adherents.com/rel_USA.html
For 2004, it was estimated that this included 71,796,719 Catholics.
So, subtracting one number from the other, there would have been about 152,641,240 non-Catholic Christians in 2004. Subtracting 1,200,000 Orthodox believers, (see source below), this would leave 151,441,240 “Protestant” believers (not sure if this would include folks I would not consider to be Protestants, such as the 3 million LDS and the JW believers).
hirr.hartsem.edu/research/orthodoxsummary.html
Also, for 2004, it was estimated by
adherents.com that there were:
47,744,049 Baptists
19,969,799 Methodist/Wesleyans
13,520,189 Lutherans
7,897,597 Presbyterians
4,870,373 Episcopalian/Anglicans
3,659,483 Churches of Christ
1,944,762 United Churches of Christ
1,560,890 Assemblies of God
Using round numbers, this is approximately 103 million people or approximately 68% of the Protestants in America. (Much of the rest are in Pentacostal or Charismatic Churches). The Churches encompassing these 103 million people are divided into about 20 denominations.