12 pitch classes sounded singly
  66 two-voice chords (not counting inversions)
 220 three-voice chords (" " ")
 495 four-voice
 792 five-voice
 924 six-voice
 792 seven-voice
 495 eight-voice
 220 nine-voice
  66 ten-voice
  12 eleven-voice
   1 twelve-voice uberchord
----
3434 distinct sonorities

Some of these will be more musically useful than others: the composer decides that, and this (I believe) is the fundamental decision he/she/it makes. Theoretically, and here I'm paraphrasing Persichetti's text, any one of these permutations– let's call them chords for sake of argument– may follow another, or even follow itself. And likewise the composer will find some progressions between chords found to be more sonically useful than others.

The trick that composers face is to narrow all those choices down to a subset about which they feel strongly, whether it be for the length of one piece or for his/her/its entire body of work.

Several people here have hinted that the medium plays a role in determining the appropriate chords to use– the question of the sort of piece one would write for an orchestra versus that which one would write for a computer versus that written for a combination of carbon- and silicon-based lifeforms/musicians. Someone somewhere is using Csound to render the blues using granular synthesis, and somewhere else another Csounder would never dream of using Csound for that purpose. Taste does play a role.

For me personally, medium is not so much of an issue. Yeah, I'm interested in coming up with textures that most people haven't heard before; this is my primary reason for experimenting with Csound. But still speaking for myself, timbre means nothing if I don't have a composition underlying. There are plenty of pieces out there that consist of seemingly randomly-placed found sounds-- works by Varese and Stockhausen come to mind-- but that's not where I personally choose to work. I cling to the belief that we haven't yet exhausted all possibilities with the equal-tempered 12-tone octave regarding harmony, melody, or rhythm (and don't get me started on those permutations).