There is a limit, although there may as well not be one. There are only so many planets in the game, and each one is capable of building a certain amount of resource structures. Now lets say you control every single planet in the game, and all of them are maxed out. You can no longer build mines or refineries on any of your planets, but you still have an assload of resources. You build and build and build and build when finally your are out of resources, and you can't build any more mines or refineries. That would be a limit. Although you would be able to alter the limit by scraping buildings and building more mines or refineries and still continuing construction, but say you do that then eventually you would not have any shipyards, or training facilties, or construction yards, and this would serve as a limit. Those aren't official limits, but you can consider them to be.