There may be cases where -- if you think of US newspaper markets, which tend to be local or regional monopolies -- it's an internal plurality point, really. Because they are monopolies, they have to represent a wider number of views. Secondly, there's an economic theory called Hotelling's effect, not because it has anything to do with hotels but because the economist who advanced this idea was called Hotelling, which suggests that in certain sizes of market -- it may be five or six players -- you have a tendency to cluster around the centre of the market. This is usually illustrated with the idea of two ice cream salesmen on a beach. They end up back-to-back selling vanilla, whereas if you have one, they might have a wider range of flavours and they might walk around the beach.
But there are good economic reasons why the five major news networks in the US were all covering the OJ Simpson trial continuously, which is not diversity, and you do not always solve that by having more. You can have an increase in diversity having less.
The same is not true in relation to the sufficient plurality of persons, which I would argue is a proxy for this opinion-forming power. I can provide a reference to our longer paper, where we develop that point, and there are some references there.