The limit is 300 neighbors. If you've reached it, you can add players as Facebook friends instead. They will automatically appear in your game.
Please note that incoming and outgoing neighbor requests are also counted towards the limit, so if you suddenly can't add more neighbors, check these sections and clean them up.
Please note that incoming and outgoing neighbor requests are also counted towards the limit, so if you suddenly can't add more neighbors, check these sections and clean them up.