system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a symphonic orchestra work?

Is there a standard set of instruments or does it vary? 

How is the amount of each instrument determined?

What is the reasoning behind how every group of instruments is positioned?

Can each musician hear the sound of their own instrument?

Isn't it crazy loud when you're there in the middle?

If someone makes a mistake, will the others spot it immediately? 

How do so many people manage to find time to rehearse together? (I'm guessing not all orchestra are full time pros?) 

How much do you need to know about other instruments than yours?

And a gazillion other questions!


**Edit**: thank you so much everyone for taking the time to reply and for the insight. This is amazing. I am reading every comment!
gpt
1. it varies.  there is a "classic" configuration for symphonic orchestras, baroque orchestras and so on.  The symphony orchestra is most typically from about 75-90, but...actual results vary a lot because everyone is trying to be interesting (in that classical music "radical is having 100 members of your orchestra! kinda way).  The quantity of each of the instruments varies within some general guidelines and in general the world of _classical_ symphonic is well aware when you're pushing the boundaries that are as much about tradition and norms as they are about composer-driven-rules.  You'll almost never see anything but the 1st violins to the left of the conductor, but...you'll see lots of variation in the placement of the bass for example and percussion are virtually always in the back, etc.

2. the number of players in each instrument/section varies.  You'll not have lots of timpani players, and you won't have more violas than violins, and you'd never have as many trumpet players as cellists and so on - this is about balancing the sound.  There aren't rules, but volume levels drives much of this!  You've got to have enough to have a "section" rather than a single voice (some parts call for a single-voice/solo of course) so you'll not see fewer than 3 of a a non-percussion instrument very often).

3. positioning is based on tradition and some practical matters.  the first violin (principle of the orchestra) has a role that needs them at the front near the conductor, but beyond that the basic configuration is premised on getting section leads near the conductor and then distributing sound to get the right blend.

4. You can _usually_ hear your own instrument, yes.  And...when you're playing "hearing" is a complex set of inputs that range from ear to touch and the physical aspects of engaging with the instrument - you have more cues to allow you to hear (a bit like how you can hear someone's words better when you can also see their lips).  You certainly can't hear yourself as well as you can when no one is around!).

5. It's loud.  Not crazy loud, but some instrumentalists do wear earplugs.  

6. You can spot some mistakes and not others.  If the oboe player fucks up during their solo, you hear it.  If the fifth row back second violin player makes a boo-boo it may go unnoticed by someone in the bass section.

7. In professional orchestras it is a _job_.  Their employment agreement (and union agreement in the U.S. anyway) dictates the amount of rehearsal time and often the time of day it occurs.  If you're top-notch symphony (e.g. boston, SF, philly, NY) you make pretty good money such that you may not need to teach.  Second tier orchestras don't pay as much and don't practice as much!

8. Being generally knowledgeable about instruments is a good sign of musicality.  The reality however is that the musicianship demands of any single instrument in the orchestra are so intense at the levels of major symphony orchestras that specialization and mastery are far more important than general knowledge.  (I find this to be a bummer, and one of the things that had me move from classical to jazz violin!).