This study on American Protestants was compiled in 1952:
"U.S. Protestants belong to 250,000 congregations, count more than 90,000 ministers and own approximately 200,000 church buildings... But still, you ask, 'What is a Protestant?' For there are 222 Protestant denominations, ranging from the Methodists, with 8,792,569 members, to tiny sects like the Primitive Friends which, at a recent count, had a flock of 13."
"The multiplication of denominations in America has sometimes been looked upon as a reflection upon Christianity. It is actually a tribute to American freedom and tolerance."
"Protestantism binds the individual to God Himself - not to any human institution."
You might enjoy reading the Pew research data on Christian denominations within the United States...