Balance has nothing to do with numbers. It's all about influence.

The nature of the Force is that the dark side just happens on its own in everyone, and people consciously choose the light to keep it in check—balanced out. People consciously choosing darkness as the Sith do is unnatural and causes imbalance.

Well, according to Darth Bane's logic, the amount of Sith being limited to two would make these two the more powerful, indeed, since the potential of the dark side of the Force wouldn't be diluted the way it would be if it was spread out over several other dark side users (hence the Rule of Two).

But from what I understand (thanks to several brilliant interpretations of other Wook users) is that just any Sith's existence "taints" the Force, disrupts the balance. The dark side is a corruption of the Force while the light itself is the balance. So, yeah, technically, already two Sith would unbalance the whole thing. But it's a bit complicated, indeed

I interpret “balance in the Force” as light keeping darkness in check. However, this “light” it's not necessarily a side in the Force because the Force has no sides, the Force just is. The light represents all positive feelings and emotions we, sentient beings have, as well actions. The dark is the exact opposite of it. Therefore, neither the light nor the dark can be eliminated because both of them are part of our nature, yet there can be an equilibrium between the two once we follow the way of the light, being mindful of our inner darkness but keeping it in check, developing self-control over the course of our lives in all areas. Just like it is in the real world. Mankind society might not be perfect and kind of chaotic, but if we still here is because we have had self-control enough and thinking with the philosophy of the light throughout our history.

In addition to this, the mystic representations of Ashla, Bogan and Bendu can be paralleled with the Hindu triad of gods: Brahma, Vishnu and Shiva. Ashla connected to Brahma, representing creation; Bendu connected to Vishnu, representing preservation; Bogan connected to Shiva, representing destruction. This triad illustrates the natural cycle of life - birth, grow/maturation, death - in all aspects, each one of them with their specific moment, thus forming a “balance” in all things in the universe. This same reasoning is personified in The Ones: the Daughter (Brahma), the Father (Vishnu), and the Son (Shiva). They represent what the true nature of the Force is - an endless cycle of creation, preservation and destruction. Everything comes from the Force and everything will return to it. As Yoda once said: “luminous beings we are, not just matter”. It also shares a similar principle with Abrahamic religions: “from the dust you came, to the dust you shall return.”

Notwithstanding, the Force also parallels the Judeo-Christian deity, on having a will and sending to the Living Force, material incarnations of its power and nature, that were created on purpose to fulfill its will and generally were prophesied by Forceful beings. The Chosen One is probably the most blatant example of this. The infinite cycle of creation/preservation/destruction may be represented as metaphor in the four seasons of the year (specifically in lands of the northern hemisphere). Spring being creation (Ashla/Daughter/Brahma), Summer being preservation (Bendu/Father/Vishnu) and both Autumn and Winter being destruction and death (Bogan/Son/Shiva), yet Autuum represents more a period transition. This cycle represents the natural cycle of all living things in the Living Force, however Darth Sidious interrupted this cycle, perpetuating the atmosphere of death and destruction with his Galactic Empire. Anakin (yes, Anakin, not Vader) brought balance back when he eliminated the element that was perpetuating that “winter” through an act with the nature of the light side - saving his son in the name of love. Consequently resuming the natural cycle of life bringing the “spring” again to the galaxy.

Of course, there are much more elements into this to explore - Jedi, Sith, Je’daii, Abeloth, Bendu, Sith’ari, Force Dyad, etc. - but they are just more examples that follow and are based in the principle I explained above and in which my hermeneutic about the franchise is funded.