D&D has too many weird and inconsistent rules for me to possibly begin to answer this question
There are a lot of different variables at play to determine how high someone can fall before taking damage as they hit the ground. These include but aren’t limited to ground type – stone, earth, sand or water softening impact-strength face down or side up – it matters! The kinds of clothes worn, and armor might also make a difference. Some say the damage might continue to rise higher than 20d6 with time on sliding down an ice floe in full plate mail for instance.
The rulebooks actually show examples that I think help answer your question the best. In a transition from one type of surface (floor) to another (ceiling), there is no set number for maximum damage. As long as the fall is within your maximum possible distance and doesn’t exceed it, you just take the highest die roll and that’s what you damage by.
In D&D 5e, falling damage can go up to 25d6.
The problem is that falling is a TABLE-DRIVEN process. There are hit points and resistance thresholds; so your best bet will be to do some research on the rules for people who fall from a height. That way you might find the answer to your question within D&D!
The Damage Multiplier shown in the table is should not be multiplied by any other dice.
I would recommend looking it up on this site, because there are a lot of incorrect answers out there. I can tell you that technically, no, the 20d6 max does not come from 5e D&D rules because Damage Multipication does not apply to falling piles of objects for balancing purposes or for example even when using an engine such as Roll20 as per below excerpted from their site forums and also this article.
“Also note that damage modifiers and multipliers don’t apply to falling (piles) – they’re just considered “one object,” so anything besides Basic Attacks will only do 1 point.”
The answer to this question is “no.” Falling damage maxes at 20d6 due to the limit of six dice per an attack.
There are rare circumstances where falling can inflict more than 20d6 of damage, but they’re extremely specific and probably not what you’re looking for.
For example, imagine that a creature has just fallen off a very high place, such as the top of an impossibly tall tower. The fall is long, but this creature has a speed of 20ft . This means it falls at half speed for 10d6 damage.
Then, during the same turn it landed, the creature moves to the far end of that platform to safety. If there are no creatures waiting or standing at the end of this platform (which there likely wouldn’t be) the creature can take its turn. It moves to the end of the platform, and takes another jump back towards where it fell from. This means that in one turn, the creature can move 10ft (falling at half speed for 5d6 damage), then 30ft (a full move action).
Since this is one turn, and the creatures actions aren’t limited to one per round (it could potentially take a bonus action and attack), then this means that falling 1d6 (half speed from 10ft) plus moving an extra 30ft is equal to exactly 5d6.
In other words, there are circumstances where falling damage can be increased beyond six dice. However, these circumstances are specifically about lengthening the fall to increase damage.