The classical response to non-dual philosophy is: I can see you are one thing. I can see I am another thing. There are two things in this room. The claim that there is fundamentally only one thing seems to require either very bad mathematics or very special pleading.
This is a fair objection. And the traditions that have grappled with it most seriously are the ones that have not tried to make it easy.
Shankara's Advaita Vedanta: the multiplicity is apparent, not real. Like the space in a room and the space in a pot — there appears to be divided space, but the space itself is undivided. The division is in the containers, not in the space. When the containers are removed, only space remains. The individual selves are like the room-space and pot-space — divided in appearance by their limiting conditions, undivided in their essential nature.
Kashmir Shaivism's response is different and, for many people, more satisfying: the multiplicity is real. But it is Shiva's own real multiplicity — a genuine expression of infinite Consciousness through infinite forms. The ocean analogy: the ocean does not become many oceans when it forms waves. The waves are real waves. The ocean remains one ocean. The multiplicity and the unity are simultaneously true at different levels of analysis, neither one cancelling the other.
Abhinavagupta's most precise formulation: Shiva's power of Vikāsa — expansion or manifestation — produces genuine differentiation from an undifferentiated ground. The many are genuinely many. The one is genuinely one. The relationship between them is not the relationship between appearance and reality — it is the relationship between the whole and its real expressions. The expressions are real. The whole is also real. And the whole is not diminished by the expressions, because the whole is infinite.
The practical implication is not primarily metaphysical. It is about how you relate to difference. The person who experiences multiplicity as a problem to be resolved — who is always trying to reduce other people to a single framework, to find the one right answer that eliminates the genuine plurality of perspectives — is working against the grain of what reality appears to actually be.
The person who can hold the multiplicity and the unity simultaneously — who can appreciate the genuine otherness of other people while recognising the shared ground — has access to a quality of intelligence and compassion that the reductionist cannot reach.
The many are genuinely many. And they are also, in a non-obvious way that is worth investigating, one. The investigation of how both are true is called philosophy. The direct recognition of how both are true is called awakening.