Why did the orchestra grow from a few dozen players to over a hundred?

The orchestra grew in steps, each one adding players, instruments or audience. In Bach's day an orchestra was not a fixed thing. Sizes and instruments differed widely between European regions: at Köthen Bach had an ensemble of up to 18 players, while Corelli's orchestra in Rome ranged from 35 to 80 players and swelled to 150 for special occasions.

By the Classical era it had settled into a pattern: a modest string section and a core of paired oboes, flutes, bassoons and horns. Beethoven, working after Haydn and Mozart, is credited with the "standard complement" of doubled winds and brass, a model symphonies kept to for decades after his death. He also pushed it outward, adding a third horn in the Eroica and piccolo, contrabassoon and trombones for the triumphant finale of the Fifth.

Then machinery joined in. Valves for brass instruments, invented in 1815, began a series of technical innovations, and these led Berlioz to write the first systematic treatise on using instrumental sound as an expressive element of music. Wagner's operas called for even more, Das Rheingold alone asking for six harps, and the typical orchestra peaked at as many as 120 players in the works of Wagner and later Mahler.

Who the music was for changed too. In the Baroque era an orchestra might live at an aristocrat's estate. In the Classical era, as composers looked to the general public for support, concerts moved into halls where anyone could buy a ticket.