Good advice Flamestrike. But the encounters I'm creating are meant as BBEG encounters and I'd like to assume that the PCs make it to that encounter with full resources, and that the encounter has the potential to kill them.
But... they really shouldnt be reaching the BBEG with full resources. Thats what his minions are for. Its fair to assume Orcus, Tiamat, Grazzt etc have the resources available to them that simply walking up to them and attacking them is all but impossible (even for high level PCs) to achieve. I mean these guys have Archmages on staff, demiliches, liches, death knights, whole armies of demons/ devils/ dragons etc and hundreds of thousands of years experience in fighting PCs and cosmic level threats.
Its also really boring to do. High level play should be something more intresting than mindless rocket tag.
Mechanically, 5E is not based around the challenge of the individual encounter. Its based around the challenge of the full adventuring day; marshalling your resources over several encounters, selectively using and rationing those resources (spell slots, hit dice, hit points, action surge, sup dice, channel divinty, smites etc) over those encounters.
The games math, encounter difficulty, class balance, and mechanics revolve around this assumption.
If we're discussing how 5E handles high level play, then we should be looking at it within this paradigm.
I realize that 5e isn't built with this in mind, but 6-8 encounters a day for 20th level characters just doesn't "feel right". I'm of the mind that at the very high levels there should be only 1-2 encounters a day. Any more and it somewhat devalues the monsters.
OK, but if you run it this way, dont expect the game to work. As you point out, the game isnt built to handle 1-2 encounters per day.
More correctly, it is designed to do this, but you need to use the 'gritty realism' variant to make it work properly, and even then, the true test is to see how the PCs are faring after 3-4 such days in a row with no long rests in between.