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If I have a Cmaj13 chord, it uses 7 notes. Basically all the notes of the scale. In a previous question, I was told that you rarely play all 7 notes for a 13th chord. So how do you know which ones to play, do you just randomly pick notes from it? Also if I choose certain notes it might just be a triad. So how do you get that defining voice that would make it a 13th, or 11th, etc when you just use a subset.

Conventionally, the most often skipped notes in any remotely extended chord are the 5th and any degrees above the 7th that aren't included in the chord name. For example, V13 chords in classical music almost never contain the 9th or the 11th.

One major exception to these conventions is the 11th chord, where the 11th and the 3rd often are assigned notes a semitone apart (e.g. for the G11 chord, the 11th is a C while the 3rd is a B). It is for this reason that the 3rd of an 11th chord is often skipped instead of the 5th.

Anything over a 7th must contain that 7th, be it major 7th or minor (b) seventh. It's been well established that the 5th (perfect) can be dispensed with, as its sound is contained in the root note.

For me, 9ths must have 1,3,7 and 9 - although if there's an instrument playing the root, such as a bass, it can be pared down to 3,7 and 9. Often in jazz, a four note chord - such as G7 - gets played with 3 and b7, just those two notes. Root probably provided by bass, and 5 not needed.

11ths are above 7, so need to have that. Often there's no need to play the 9th, and the 11th comes in rather like a 4th, so 1,3,(5),7,11.As 11 is the same note as 4, an octave higher, it doesn't sound good played as a 4.

On piano, it's very easy to voice any of these chords with or without the intervening intervals. Like the sound? keep it in. Dislike it? remove the offending note/s. On guitar, it's quite a different scenario, as often there are 5 or 6 notes to play, but fingering won't allow all. That's when it's more important to consider which get omitted.

13ths would contain pretty well everything, and the kitchen sink. The essentials for me are 7 (of course) usually b7, and 13, which is an octave copy of a 6th. So - 1,3,(5),7,13.

Obviously if the 5,9 or 11 is # or b, they need to be there, but otherwise, there's often little point in crowding things in.

This answer is not entirely containing all cases but provides some examples and solutions:

As you may know extended chords are to find quite often in a fith fall sequence: You probably have noticed that here the 3 becomes the 7 in the next chord and the 7 can be the sustained 4 or falls a minor second to 3, while the tonic becomes the new 5: d,f,a,c => g,b,d,f => c,e,g,b => f,a,d,e etc.

The voicing rules in classic and in jazz and pop music are similar: keepin a note of a chord in the same voice by let drop the fifth or third to play the extended chord notes.

Notice that you have in this case the 5th fall in the bass line and two pairs of descending thirds guiding downsteps (in a progression of secondary 5th aswell in the diatonic 5th fall sequence and also with extended chords of 79.

If you look at the symbol Cmaj13 in sort of nicely reveals what is essential to voice this chord. You obviously need tonic for 'C', then to reflect 'maj' you need the notes that define 'maj' quality of the chord - and that's maj3 and maj7. Then you have '13' - the defining extension. All other notes are not neccessary to get a maj13 sound.

So to sum up, you need tonic for obvious context, you need to spell out the basic quality of the chord (maj, min, dominant, maj7) and you need the highest extension.

As for the rest of the notes there's no right or wrong - it all depends on the context, also it might be dictated by limitations of the instrument (eg. you sort of have to drop some notes to play Cmaj13 chord on 6 string guitar)