Number of parts in orchestral music

I am studying harmony with Walter Piston book. All its examples are 4 part. But how does it scale to orchestral scores? How many parts do orchestral compositions use? What are the min and max number of parts usually employed (simultaneously and not)?

Another questions:

- What are the most common instruments that can play 4-part music alone besides the piano?

- Do common-period composers use the harmony theory explained by Piston among all the parts of an orchestra?

Sorry for my ignorance. I've just started learning

The numbers of parts in orchestral music can be anywhere from four (string orchestra without seperate double bass voice) to thirty or even fourty and more. There's not really an upper limit. In such large scores however, most of the time not all instruments will play, and even if they play together several instruments may double the same voice. But in late 19th century and 20th century music there can be very well passages where a great number of instruments play a different voice at the same time.

The most common instruments that can play four parts at once would be all keyboard instruments (organ, harpsichord, celesta, etc.), accordeon, and to a lesser degree most plucked instruments (harp, guitar, etc.) and xylophone/marimba/vibraphone.

I've never read the Piston book, so I can't really answer the third question, but since harmony is about the simultaneity of sounds, you can only really speak of one harmony if it applies to -all- voices that play at the same time. In general, the same harmonic rules apply to all parts, they just have different roles: A double bass will play bass notes and will rarely play a note that doesn't fit into the harmony, whereas a flute playing the highest voice may often "fill in" the space between two harmonic notes with unharmonic ones, to create a more fluid melody.

Most tonal harmony is based on only 3-4 notes at once. If a orchestral piece is tonal, the chordal texture will be only 3-4 different notes spread out over different octaves. The "bells and whistles"-melody, counter melody, bass line, and other embellishments- added on top of the chordal texture is what makes orchestral music so complex.