"Core" refers to a single unit or single region.
If the 8 or so chambers were spread out evenly throughout the ship, then there would be no reason to refer to it as a core. However, that is not the case. Yes, there are what appear to be 8 or so individual dilithum chambers, as they are referred to in the film, but since they are all located within a single space, that area would be referred to as a "core".
If the 8 or so chambers were spread out evenly throughout the ship, then there would be no reason to refer to it as a core. However, that is not the case. Yes, there are what appear to be 8 or so individual dilithum chambers, as they are referred to in the film, but since they are all located within a single space, that area would be referred to as a "core".