They mean you can have one IP address, either dynamic or static. If you choose dynamic, it may change each time you connect, or may stay the same for quite some time, but it is dynamic and can change.
If you choose static, it will always remain the same for as long as your with that ISP. One reason why they charge for it. My provider does not and in fact gives 8 static IP's free of charge.
So you can have a single static, or dynamic, or a block of fixed [Static] addresses, or a block of dynamic addresses.
There are some advantages with static, that is why most providers charge extra for it, but for most users, a dynamic is just as good.