"Concerto" is itself the singular. The plural is usually taken to be "concertos," but "concerti" is more correct.
The auxiliary verb in the sentence is incorrect.The verb "were finished" should be "was finished" because the subject noun "concerto" is singular.
concerto
He only wrote one concerto, but he wrote it over 400 times.
look it up on google, improvisation like a Classical Concerto Cadenza
A concerto is a solo accompanied with an orchestra
Clarinet Concerto K.622
The plural form of concerto is concertos.
Gilberto Concerto was created in 2002.
Concerto Copenhagen was created in 1991.
Tail Concerto happened in 1998.
Concerto for Constantine was created in 2007.
Concerto Köln was created in 1985.