If you perform a Full system backup (disk image) which backs up everything on a hard drive or partition, no need for activation code again. If you have already activated the programs and then created a Ghost image of the hard disk, you will not need to reactivate any of the programs when you reimage the hard disk.
The Norton Ghost product CD is used to install the product and to start a computer by using the recovery environment. This disk is known as Symantec Recovery Disk. If you have purchased the download version of the product, download the Symantec Recovery Disk image file and create a Symantec Recovery Disk. You may also create a custom Symantec Recovery Disk for your computer. If you need to restore your computer, files, or folders using Symantec Recovery Disk (because you cannot start Windows normally), you can use the product CD to start the computer using the recovery environment.
The number of CDs/DVDs depend of the image size and data you have to backup. For 20 GB of data , it may go up to 4 DVDs(8GB each). There is also an option for backing up data in compressed and encrypted form which minimizes the storage space. Even if you use this option, some of the file formats can't be compressed further(for eg. Movie files like.dat,.mpeg). When you save backup data to CD/DVD, it is automatically split into the correct sizes if the backup spans more than one media. The size of recovery points depends on the amount of data you have backed up and the type of recovery point being stored. The speed of creating an Image depends on the destination at which you are storing it and the amount of data you have included in the image. For creating Images in CDs/DVDs may take more time as it is depended on the speed of the Writer.
Once you install and activate Norton Ghost, it is valid until you uninstall it. There is no subscription renewal for Norton Ghost Stand alone product.
For your last question, the answer is a simple one: End User License agreement. Unless you don't have valid License(for both Operating System and Other programs installed), you cannot use the same image in many computers. If you have valid licenses, you can use the same image, but you may need to reactivate those programs separately.