Apart from the four soloists, the orchestra itself consists of only nine or ten members in this performance. Here is the first page of the score:
Here is the beginning of the arrangement by Bach:
As you can see, apart from changing the key from B minor to A minor and filling in some bass lines, Bach keeps fairly close to the original. Here is a performance of the Bach arrangement. I would have liked one with video, but all that I could find were amateur performances.
One of the interesting things about this concerto, apart from its effervescent energy, is the "four soloists" who are an ensemble in themselves. This kind of concerto, very popular in the early years, is called a concerto grosso. It was soon to be largely replaced by the truly solo concerto, with only one solo instrument. But occasional examples are found later on such as a couple of piano concertos by Mozart for more than one piano, the triple concerto of Beethoven for violin, cello and piano and the Brahms double concerto for violin and cello. Examples in the 20th century are even more rare--apart from the quite different idea of the "concerto for orchestra" the example that comes first to mind is the Concerto for Piano, Trumpet and String Orchestra, op. 35 by Shostakovich, but that was likely inspired by the Baroque concerto grosso.