Well. We know for sure that majority of players will want to go Britain or become Pirates. This is a given.
Pirates issue is solved easily by the mechanics we already discussed in several Piracy topics.
Controlling country numbers is not as easy.
There are basically 3 options
1) Focus on player guilds and companies, ignoring countries, that will only provide limited content (research, blueprints, buildings in ports and trading etc).
2) Provide welfare
3) Hard lock. You can be a British National - but Royal Navy is not accepting any more applications.