there are different ways to involve a new city as mentioned in , for instance running your experiment in a new city not involved in Organicity yet. Section 3 in the Open Call instruction enumerates different ways to involve a new city (http://organicity.eu/wp-content/uploads/2017/06/OrganiCity-Second-open-call-instructions.pdf).
From a technical point of view, becoming a new Organicity (i.e. new OC site) requires the federation of the city data in the platform as detailed in . That way the federated data will be available through Organicity APIs. This federation requires to setup and maintain a server with the programs indicated in the mentioned link. It is worth highlighting that you do not need to host the platform, but just to connect your server to the core platform. The figure in  can give you an idea of this connection: Organicities are technically labeled as OC sites, and they use the federation API to inject city data to the platform.
There is not restriction to become a new Organicity, provided the ethics and privacy principles are followed (http://organicity.eu/wp-content/uploads/2017/06/Ethics-and-privacy-checklist-2017.pdf). Concerning the number of new cities, there is not restriction either.
Luis Diez (Organicity member)