As support have said, it would be better to start on a pay as you go plan, and I'd also strongly suggest that you only go up to 128kbps as it is a fair bit cheaper (especially if you're going to unlimited) and there is pretty much no difference in sound quality what so ever!! I spent years trying to find a difference, and 80kbps sound identical to 128kbps!!
With regards to your main question, you will get 30 simultaneous listeners... this means at any one time you can have 30 listeners tuned in, as soon as there are 30 and someone wants to tune in they will get a "Server Full" pop-up and be asked to come back later.
If you have one listener, and then another 2 come along... it doesn't mean you are down to 27 listeners though!! It means the capacity of your server is 30 listeners.
I hope that helps out even more than Support have already done so, and if you still have any questions feel free to post them up!!