Density & World Dynamism in MMOs

A friend of mine recently pointed me to the Fallen Thrones. An in-development MMO which will have a dynamic world. It looks similar to EVE Online with the ability of players to build and siege cities. The big difference being EVE is a space game and Fallen Thrones is more classical fantasy. In reading through the information published about the game I noticed this in the FAQ for Fallen Thrones:

3. The difference between paying players and premium paying players will be one of ease of gameplay.  Differences would include things such as a moderate increase in bank size, vault size, character creation slots, and free server transfers.

This indicates that there will be multiple servers like most MMOs but unlike EVE.This got me thinking about player density and its impact on the dynamism of the world. Adam Smith pointed out that the degree of specialization in a market is proportional to the depth of the market. This stems from two types of specialization.

First, the more people demanding a common good the more profitable it is to be highly skilled in producing that good (or a component of the good) and unskilled at everything else. These are people like the dedicated ore miners in EVE. They only exist because of the high demand for materials to craft with. Which is a result of how deep the market is. In Eve, most players don't make their own ships or weapons, they buy them from other players.

Second, the more people, the more varied the tastes and thus the more likely there will be enough people to support so obscure profession. Fallen Thrones aims to support sieges, which implies that people will specialize in siege equipment. This is only profitable if sieges happen with relative frequency. Given the restrictions placed on the number of PC cities per realm (on the order of 10), there will have to be many realms and many players in each realm to support this profession.

So to have truly dynamic player cities and intercity trade, there needs to be enough players to support high levels of specialization. The big question is, will each server have enough players to support the features the developers are building? How many active players do you think there needs to be on a server to have such a dynamic world?