Canon EOS REBEL T8i Advanced User's Manual page 641

Auto Reset
When you wish to restart the file numbering from 0001 each time
the card is replaced or a new folder is created.
When you replace the card or create a folder, the file numbering restarts from 0001 for the
new images saved. This is useful if you want to organize images by cards or folders.
If the replacement card or existing folder already contains images recorded previously, the
file numbering of the new images may continue from the file numbering of the existing
images on the card or in the folder. If you want to save images with the file numbering
starting from 0001, use a newly formatted card each time.
File numbering after replacing the card
Card 1
Card 2
(1) File numbering is reset
File numbering after creating a folder
Card 1
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