"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.
Concerto Köln was created in 1985.
Concerto Moon was created in 1996.
Compaq Concerto was created in 1992.
Concerto DSCH was created in 2008.
Concerto Gate happened in 2007.