Examples: Masses/Requiems (Bach, Mozart, Schumann), Scriabin's Mysterium etc.
When I had to write simple four-voice chorales for music theory, I struggled to keep track of all the voices - how is it even possible to know what you are writing sounds like? This is under the assumption that 1) no perfect pitch 2) not at a piano to test chords as you write.
Of course, you write down the chords at the bottom of the staves in roman numerals, but still, it makes my head hurt. Did these composers write down chords in roman numerals as well? Especially for polyphonic works (Bach) - how do you know if the "clash" from suspensions etc are correct? In "Gloria" from Bach's b minor mass, I noticed how when the woodwinds play the opening theme, all four voices are playing distinct "accompaniment". How? That's half a dozen independent voices to keep track off.