“One useless man is a shame, two is a law firm, and three or more is a congress” is attributed to John Adams (1735-1826), but he never said it. The real authors are the people behind the musical 1776, originally produced on Broadway in 1969. The John Adams character says: “I have come to the conclusion that one useless man is called a disgrace; that two are called a law firm; and that three or more become a Congress!”
   
In the 2000s, many blogs and websites (and even books!) have falsely attributed the witty theater line to the historical John Adams.
     
 
Wikipedia: 1776 (musical)
1776 is a musical with music and lyrics by Sherman Edwards and a book by Peter Stone. It is based on the events leading to the writing and signing of the United States Declaration of Independence in Philadelphia, Pennsylvania, in 1776.
 
The musical was produced on Broadway in 1969, running for 1,217 performances, and was made into a film of the same name in 1972. The show was nominated for five Tony Awards and won three, including Best Musical.
   
Wikipedia: John Adams
John Adams (October 30, 1735 – July 4, 1826) was an American politician and the second President of the United States (1797–1801), after being the first Vice President (1789–1797) for two terms. He is regarded as one of the most influential Founding Fathers of the United States.

Adams came to prominence in the early stages of the American Revolution. As a delegate from Massachusetts to the Continental Congress, he played a leading role in persuading Congress to adopt the United States Declaration of Independence in 1776. As a representative of Congress in Europe, he was a major negotiator of the eventual peace treaty with Great Britain, and chiefly responsible for obtaining important loans from Amsterdam.

Adams’ revolutionary credentials secured him two terms as George Washington’s vice president and his own election as the second president of the United States. During his one term as president, he was frustrated by battles inside his own Federalist party (by a faction led by Alexander Hamilton) and the newly emergent bi-partisan disagreements with Jeffersonian Republicans. During his term he also signed the controversial Alien and Sedition Acts. The major accomplishment of his presidency was his peaceful resolution of the Quasi-War crisis with France in 1798.

After Adams was defeated for reelection by Thomas Jefferson (at the time, Adams’ vice-president), he retired to Massachusetts. He and his wife Abigail Adams founded an accomplished family line of politicians, diplomats, and historians now referred to as the Adams political family. Adams was the father of John Quincy Adams, the sixth President of the United States. His achievements have received greater recognition in modern times, though his contributions were not initially as celebrated as those of other Founders.
   
“1776” musical (transcript)
ADAMS: I have come to the conclusion that one useless man is called a disgrace; that two are called a law firm; and that three or more become a Congress!
     
The Cleveland Memory Project
“1776” captures colonial capers
By Toni Mastroianni
Cleveland Press February 2, 1971
“1776” is a wonderful show; a pleasant, satisfying and superb production that leaves you sorry it is over.
 
The musical play opened at the Hanna last night where it will remain for three weeks. I wonder if the cast realized how unusual it is to take a curtain call with most of the Hanna audience still seated. Maybe others also shared the feel-successful as the show has ended because they stayed on and applauded rather than rushing for the exits.
(...)
The tone for the evening is set as John Adams, brilliantly played by Patrick Bedford, steps out on the stage to inform us that one useless man is a disgrace, two a law firm and three or more a congress.
   
Google Books
Distances:
Between people; between people and their government; and between points on a map
By Fred Meeker
Burlington, IA: Craftsman Press
1988
Pg. 88:
I had wanted the lead role of John Adams, who begins the play by saying something about his having come to the conclusion that he calls one man who knows nothing a fool; two a law firm; and three or more a Congress.
   
Google Books
The Quotable John Adams
Edited by Randy Howe
Guilford, CT: Lyons Press
2008
Pg. 207:
In my many years I have come to a conclusion that one useless man is a shame, two is a law firm, and three or more is a congress.