The world of "classical" music is extremely diverse. In general, and collectively, it spans about 1,000 years when it is all lumped together. Its evolution has taken it from choral music without accompaniment, and then with sundry instruments, to just instruments and then various sizes of ensembles. This evolution followed the development and standardization of instruments and harmony.
Throughout this layering of era upon era, so too developed the patrons of the music. In the beginning, it was primarily the reigning churches of the day and location. With the development of the upper and middle classes and individuals who found great financial success, the patronage shifted and along with it, the messages of the music, the instrumentation, the harmony, and the location—the venue.
With the coming of the upper class and its newly developed sponsorship, came private concerts in people’s homes. These were held in large rooms within the homes, and thus the number of instruments used were, by necessity, few. The music performed in these settings had to have a more intimate character to suit the room and the small audiences. In those days it was common for the affluent to have rooms with designated names, such as the salon (seating 30), parlor, lounge, gallery, drawing room, ballroom, and chamber—which might have a seating capacity of up to... depending upon the wealth and the size of the house, but significantly less than a concert hall. Because of the venue and circumstances, this music became known as “chamber music.”
Today, famous ensembles—such as string quartets—can be seen/heard performing on large stages in large concert halls because the demand to hear these famous musicians is equally large.
To truly appreciate chamber music at its best is to experience it in small concert halls or rooms, with small audiences, and with close proximity to the musicians.
Typical ensembles in these venues are string quartets, string quintets, string trios, woodwind quintets, and combinations of woodwinds and strings, but usually not exceeding 10-12. Sometimes very small orchestral-type of musical orchestrations are provided by composers, but the atmosphere – the ambience – for the performance is intended for a setting of an intimate nature.
A good resource for learning more about “chamber music” can be found at https://www.britannica.com/art/chamber-music
the Music Master