I didn't find a perfect solution, but it goes like this: it doesn't matter how many components an entity has, as long as all components are stored in correspond

I didn't find a perfect solution, but it goes like this: it doesn't matter how many components an entity has, as long as all components are stored in corresponding arrays. So, you have positions array, velocity array, etc. Those arrays contain the data ideal for consumption by corresponding systems. The problem here lies in linking those separate components (i.e. indices in arrays) to entities.