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The number of SETUP-*.BIN files to create for each disk. If this is 1 (the default setting), the files will be named SETUP-x.BIN, where x is the disk number. If this is greater than 1, the files will be named SETUP-xy.BIN, where x is the disk number and y is a unique letter.
One reason why you may need to increase this from the default value of 1 is if the size of your disk media exceeds 2,100,000,000 bytes -- the upper limit of the DiskSliceSize [Setup] section directive. If, for example, your disk media has a capacity of 3,000,000,000 bytes, you can avoid the 2,100,000,000-byte disk slice size limit by setting SlicesPerDisk to 2 and DiskSliceSize to 1500000000 (or perhaps slightly less, due to file system overhead).