It’s not so about having maximum number of good builds rather builds that are to be tried by the player.
A designer’s goal has to be the player to experience as many builds and gameplay styles as possible.
If we take PoE for example and assign 100 million as theoretically possible builds aka B1 = 100m while comparing it to D3 with say 100k builds aka B2 = 100k, we’d expect PoE to have way more gameplay variance (since B1 > B2), but that’s not the case due to the reasons I mentioned above - most players doing easy content; most players going after meta/copying builds; builds playing the same way and not leading to new gameplay.
So, in the end it turns out that D3 offers more gameplay variance than PoE even while having thousand times less builds.
This is something the D4 devs have to consider while designing the game: It’s not the optimal number of builds that matters, it’s the optimal number of unique gameplay that matters.
To explain it even more simple with a real life example: Say one guy has 10 cats and they all behave similarly - laying lazy all day long, but another guy has a single cat that is very active displaying many unique types of behavior. That one cat is more fun to have than all those 10 other cats.
This is why Diablo 4 have to either:
1] Narrow classes from start (with choosing a clan) or narrow classes at level 50 with weapon specializations/professions
OR
2] Implement variables shuffling per account