When there's no heir to the throne and the current monarch dies, a personal union will form. If the country doesn't have royal marriages or a claim from another country, it will get a Regency council. Sometimes if you have a royal marriage with a country, you can form a PU with them. Just note that you will get someone to PU with them instead of you, because prestige matters. So the higher the prestige of the country has the royal marriage with/claim, it will choose that one.
Also, I've begun to have colonies in South America, so I hope the Carribean won't be taken by Castille.
As of July 30, 2011 11:15 PST, there's a total of 32,301,770 people living in the world in my English campaign.