A string quintet has 20 strings - 2 violins, 2 violas, 1 cello

of 2 violins, 1 viola and 2 cellos

A quintet is usually comprised of 2 violins, 1 viola, 1 cello, and 1 double bass. I have seen variations of that though. One example is a quintet by Schubert (op. 163), which calls for 2 cellos--the second one replacing the double bass.

4 violins, 2 violas, 1 cello and 2 double basses. If you listen to it carefully, there is also a harpsichord playing in the background.

A concertino, or small group of soloists (usually 2 violins, a harpsichord, and a cello), contrasts with the cocerto grosso, which is the whole string orchestra and its ripieno players.

yes

2 x 279 = 558