Is there a guide to how to rebuild the battery pack? How many cells does it need?

You can also purchase 28 New modules from Toyota. Retail price is probably near $2100 depending on where you live.
(Less if you have a wholesale account?)
They come in a case with all the bussbars, etc, but there is additional assembly needed.

 

That isn't much different from buying a whole battery. I have even seen complete batteries for less than that from some Toyota dealers, but perhaps these are refurbs rather than new? This page for instance doesn't say explicitly, usually parts from dealers are new unless otherwise specified:


Toyota Prius Battery assembly, hv supply. Electrical, wiring, cable - G951047031 | Toyota of Lake City, Seattle WA

 

What you get from the dealer is basically just the modules. Many parts need to be used from the old battery pack.
If you are DIY @2k1Toaster here sells a kit or new cells for $1600 delivered. That basically gives you a new battery pack with improved cooling. The link is in my signature.

 

The battery you and Eric are talking about is the same thing. The factory battery is not complete. You have to use things from your old battery to complete it.

Eric has done so many he could probably tell you what parts you need from the old battery off the top of his head.