Ok, so my understanding of foreign trade routes is that you need to be able to link your capital with the trade partner's capital, up to a maximum of the number of trade routes your ports can support (5 for Great Britain, at the start of the campaign).

However, when setting up diplomatic relations in turn 1 I find I can actually trade with many more than 5 factions, so far I have:

United Provinces
Austria
Hannover
Denmark
Thirteen Colonies
Muratha Confederacy
Mughal Empire
Ottoman Empire
Venice

I don't get why I can trade with this many factions. Is this a bug??