So, after conquering all the galaxy empires, vassalazing them and then integrating or taking their planets to me, my borders now represent like 80% of the galaxy.
In the early and mid game, you could easily manage more than 5 planets because there's not too much stuff to deal with and the game hasn't reached its laggy part, which happens indoubtedly after a certain point, but the point in controlling 5 core in the start of the game, i believe, is that you don't really need more than this, actually.
Really, with 5 spaceports easily acessible through the user interface, you can manage to build and control large amounts of ships, armies and resources etc...
I understand why some players would like to control more. Me too, i wish i could micromanage all of them actually, but this would reallyturn the game into a micromanagement type of game, and you wouldn't be able to really enjoy all of the game's features because you have to spend all the time doing micromanagent.
I do believe though, that controlling like 10 planets would be easy and would have more space to spread your core planets throughout the galaxy, making the core planets specially useful.
But i must admit that the game isn't very well improved in the performance sector, making it impossible to deal with so many planets. Even when your planets are inside the sectors the game will stutter a lot.