Canon IXUS 50 - Manual (Page 114)

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About File Numbers and Folder Numbers
Images are assigned file numbers from 0001 to 9900, and folders are
assigned folder numbers from 100 to 998 (Folder numbers cannot
contain 99 in the last two digits).
Folder File Capacity
Each folder usually contains up to 100 images.
However, since images shot in continuous shooting and Stitch Assist
mode are always saved into the same folder, a folder may contain
more than 100 images. The number of images may also exceed 100 if
a folder contains images copied from a computer or images shot with
other cameras. Please note, however, that the images in folders
containing 2001 or more images cannot be played back in this
camera.
Folder numbers cannot contain 99 in the last two digits