The maximum result for a skill check is at least 169
The Rules
In order to maximize a skill check, we must use the following variant rules:
The Party
Our party, from Theros, consists of four characters:
- A bard 15 -------------------------------------------------------- (for Bardic Inspiration)
- A Circle of Dreams druid 6/ Wild Magic sorcerer 6 ---- (for pass without trace, Bend Luck, Hearth of Moonlight and Shadow)
- An artificer 7/Peace domain cleric 1 ----------------------------------------------------- (for Flash of Genius and Emboldening Bond)
- A hobgoblin Oracle rogue 1/ranger 10/The Fiend warlock 6/Any 3 ------- (for Oracle's Insight, Expertise, Save Face, Hide in Plain Sight, Dark One's Own Luck, Boon of Luck, and Boon of Undetectability)
Our friendly hobgoblin will be making a Dexterity (Stealth) check against a very perceptive foe.
The Gear
- A luckstone
- An Ioun Stone of mastery
- The Lost Crown of Besilmer
- 5 manuals of quickness of action
- 5 tomes of clear thought (read by the artificer)
- fresh mud, dirt, plants, soot, and other naturally occurring materials
The Calculation
- Dexterity score is 30 (or a +10 modifier)
- Flash of Genius adds 10 (Intelligence)
- a Hero Point adds 1d6 (maximum 6)
- Bardic Inspiration adds 1d12 (maximum 12)
- Bend Luck adds 1d4 (maximum 4)
- pass without trace adds 10
- Emboldening Bond adds 1d4 (maximum 4)
- Hearth of Moonlight and Shadow adds 5
- Hide in Plain Sight adds 10
- Expertise results in doubled 1d12 (maximum 24)
- Dark One's Own Luck adds 1d10 (maximum 10)
- Oracle's Insight adds 1d10 (maximum 10)
- Save Face adds 5
- Boon of Luck adds 1d10 (maximum 10)
- Boon of Undetectability adds 10
- luckstone adds 1
- Ioun Stone of mastery adds 2 (thanks to expertise)
- The Lost Crown of Besilmer adds 1d6 (maximum 6)
- Roll a 20
Grand total: 169
Epic Boons of Fate
Finally, there is an epic boon called Boon of Fate which lets you add 1d10 to an ability check of any creature you can see. With enough creatures that have this epic boon you can make the total check immense. So basically, add 10 for each character with the Boon of Fate within 60 feet. Since you could use the Minimus Containment imprisonment spell to place them in extremely small spaces, there is effectively no limit to the amount of creatures you can fit in that 1.7 million cubic feet.