This is a very late reply, and you probably have the information you need, but let me clarify the number of devices question.
UPB supports 250 Network IDs. Think of a network ID as a house code in X10. All devices on the same NID can communicate to and control each other. Then, you can have up to 250 units (Device IDs) per NID. So, while you can technically have approximately 64,000 devices, if you want devices on different NIDs to communicate to each other, you will need an intelligent controller that can "bridge" the NIDs. So, in most instances, 250 devices is a limitation, unless you have the bridging controller. I've heard of installations approaching 200 devices, but I've not talked to anyone who has put in more than 200 devices in one installation.
Most UPB devices (receivers) can be part of 16 different links (or scenes, if you prefer). You can have up to 250 links per NID and you can assign up to all 250 UPB devices to any particular link. For example, a default link that comes in most devices is Link 1, which you can consider to be an ALL ON link. The beauty of a link is that all receivers with that link in it will respond immediately to the link signal and you will not see the staggered effect that you used to see with a lot of X10 installations (depending upon the controller) where in order to get multiple lights to come on, you might have to create a macro that would turn on lights sequentially.
You can have one device associated with a link, or 250. And, each light can be set to respond differently to each link. For example, if 5 lights are associated with Link 5, one could come on at 100% with a 3.3 second fade rate, the second one could come on to 90% with a SNAP on fade rate, the third one could come on to 50% at a 5 second fade rate, and so on.
The programming is extremely powerful and flexible.