I think the confusion arises from the fact that your ISP can give you a static IP address and the fact that older Amiga TCIP/IP stacks such as AmiTCP require static IP address allocation.
If you connect to the outside world via a router, which most people using any sort of broadband package will, then it doesn't matter what sort of IP address you get from your ISP, because the IP address your Amiga uses will basically be your choice from your router's free local IP addresses.
Most of your devices on your home network will probably be fine with DHCP and just negotiate with the router for an address. If, like me, you have a few older Amigas on your network that expect a static IP address, all you need to do is use your router to reserve a few for whatever MAC address the network cards in those Amigas use. Every time they negotiate for an address, the router will see their MAC address and assign the same one each time, so as far as the Amiga's network stack is concerned, it is static and all is well.