it's an interesting question... and one that's obviously been asked many times...
I have 8 cases... in an almost permanent layout...
2 slightly angled but almost vertical with sound sources, modulation, filters and some utilities
2 almost horizontal with mostly percussion, sequencing, control and mixing (plus effects)
these add up to 18u of 188hp... and are roughly organised like a 2600
then a smaller case with some effects, pedal interfaces, instrument interfaces and es8/es6
that sits on top of 12u of a 19" rack holding 4 rows of 84hp - which is effectively an overspill rack - some audio and some video...
and then a tip top mantis which is my main video rack...
if I need to take an audio rack anywhere I can take modules out and re-purpose the mantis and the interface rack as required...
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities