Why do you have an additional router connected to the AirPort? It seems like this would, if anything, hurt your throughput.
Supporting 50 users is a strange claim that has limited basis in reality. The limiting factor in wireless routers has very little to do with the number of users, and a whole lot to do with Air Time.
Imagine you are in a crowded room, and everyone is trying to talk at the same time. Most of the conversations involve the same individual... he's the guest of honor. Fortunately, this person talks very fast, but he has an odd limitation: he can't talk and listen at the same time. If he needs to listen to someone who speaks very slowly, that eats up a lot of time that could be spent communicating with others. Additionally, as more people come into the room, not only does he have less time to spend talking with each person, but the total amount of information he is able to share with the group decreases. This is because he has to spend time in introductions, and in picking up where he left off as people come and go, and especially because people tend to interrupt each other more and more as more of them vie for his attention. It also doesn't help that the "room" has very thin walls, and outside noise often causes people to need to repeat themselves.
That's how wireless internet works. One router, even a very bad one, could support 1000 people if each person doesn't need to talk much. But if a few people want to engage in non-stop chatter — think: bittorrent, streaming media (youtube, netflix, pandora), large file downloads, skype/facetime/google hangout chats, etc — then the router may only be able to serve a very few people.
Here's the other problem: adding more routers often won't help. There's only so much air time available before you have too many people talking over each other. It's easy to reach a point where people have to repeat themselves (retransmit packets: this is handled by your network cards automatically) so often that the total amount of information shared is degraded significantly.
Supporting "50 users" is a marketing attempt to translate technical information into something the non-technical users can understand, but it really has only a limited basis in reality. It is a good rule of thumb that an access point can support 25 people per radio. The AirPort Extreme has two radios (one 5ghz and one 2.4Ghz), and so you get 50. Note that this won't help if all of your laptops only support 2.4Ghz, which is very common, and again: it depends greatly on the nature of the traffic involved.