While I dont know the exact answer to your specific question, They can be as many pieces as can be cut, and re-welded back together. This is why you see "patch panels" for sale. To replace rusted or otherwise damaged sections of the vehicle like rocker panels, fender tips, cab corners, floor pans, etc. The entire roof can be cut off, and another one put in it's place.
The most pertinent question then becomes one of economics, is this a job you can do yourself, or how much will it cost to have someone do it for you, VS. how much $ can you buy a truck without a bad roof for to begin with. HTH
