The software has several features to help you estimate how much space would be required to make your backup, and from that how many CD-R or CD-RW discs would be required. Turning a space estimate into an estimate of the number of discs cannot be done very accurately though, because the exact amount of space taken on a CD-R or CD-RW disc depends in part on the mixture of large and small files in the backup. Also, the capacity of all CD-R and CD-RW media is not the same, and the choice of packet writing software also influences the capacity of the media. It tackles this by assuming a standard media size for its calculations, and telling you the size it assumed along with the number of discs it calculated using that assumption.
The software does not consider the effect of file compression when estimating the space it would take to make a backup, or backup a drive letter or folder. Taking compression into account in any reasonably accurate way requires actually looking at the data in each file and trying to figure out how well it compresses. This would be extremely time consuming, since it would require the software to essentially read every file included in the backup from end to end and compress it to see how much space is saved. In some cases that is scarcely less work than actually making the backup. However, it is possible to make general ballpark guesses of what the effect of compression would be, by assuming that graphics, music and multimedia files do not compress at all and that everything else compresses to about one half size.