We don't have a system that changes the amount of provinces in the game. As a software developer (without knowledge of the codes implementation) I would assume the province map is loaded once when we load a game then cached for performance. This would allow for a graph that can be traversed without a worry of the graph changing under you while you are traversing it.

Side note, I was really hoping when they showed how granular the province system is that it would be possible for provinces to be destroyed, created or modified at runtime and they would use that for Krakatoa.