I guess there is no limit of the neighbours, but it will be quite hard after a while. (I couldn't say how many, as all of us still didn't get to the end)
The game probably will have other objectives, or even the tech-tree will get continued, but it is only the closed beta of the game. See in FoE, they still add newer and newer eras to the game, with new game features, I expect to see something like that. there will be probably more player interactions, as that is one of the most frequentially suggested ideas.