Even though you can only visit 18 friends within a 24-hour period, you can have many more than that. The more friends you have, the more to buy from your stores. The more buy from your stores, the quicker you earn cash. The more cash, the more floors you can purchase. You can see a list of all your friends, beyond just the 18 listed for a daily visit, by tapping the friends button. There you will also see the height of each friend's tower. As in life, it is not just size that matters but how you play the game.