The maximum average result for a Charisma check is at least 70.99
Our character is a 20th level character from Theros, and has the following classes:
- Bard (College of Lore) 18 ------------ for (Bardic Inspiration, Expertise in Deception, Peerless Skill, Magical Secrets for wish, the Lucky feat, the Actor feat and 2 ASI to Charisma)
- Cleric (Peace Domain) 1 ------------- for (guidance and Emboldening Bond)
- Any other class 1
...has the Oracle Supernatural Gift from Mythic Odyssey of Theros:
Oracle’s Insight. The gods give you flashes of insight that help you bring your efforts to fruition. When you make an ability check, you can roll a d10 and add the number rolled to the check.
...and the following Epic Boons:
- Boon of Fate
- Boon of Luck
Finally, our character has created a simulacrum of themself with the wish spell.
We will be making a Charisma (Deception) check to pass off a disguise.
Our d20 roll follows:
- Advantage from the Actor feat
- A Luck Point is expended
The average of our d20 roll after these effects is 15.49
Our personal modifiers are as follows:
- Charisma score is 20 (+5 bonus)
- Expertise in Deception (+12 bonus)
- Peerless Skill adds 1d12 (average of 6.5)
- Oracle's Insight adds 1d10 (average of 5.5)
- Boon of Luck adds 1d10 (average of 5.5)
This totals an average of +38.5.
The modifiers granted by our simulacrum are as follows:
- Bardic Inspiration adds 1d12 (average of 6.5)
- guidance adds 1d4 (average of 2.5)
- Emboldening Bond adds 1d4 (average of 2.5)
- Boon of Fate adds 1d10 (average of 5.5)
This totals an average of +17.
Grand total: 70.99
Two variant rules let us push this total a bit further, these being:
Proficiency dice replaces the +12 from Expertise with +2d12 which averages to +13. Overall this nets +1
Hero Points adds 1d6 to the check for an average of +3.5
Variant Rule total: 75.49
Technically, there is no limit if you can continue to create simulacra of yourself. The distinction here comes from the simulacrum spell which says:
If you cast this spell again, any duplicate you created with this spell is instantly destroyed.
However, if your newly created simulacrum casts wish to make a simulacrum (Epic Boon of High Magic grants the additional 9th level spell slot) it is not you casting it, so you continue to rack up simulacra. Since they each have Boon of Fate, you could add 5.5 for every simulacrum bearing no limit.