Canon EOS REBEL T7i Instruction Manual page 318

Handy Features
[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
Card-1
0051
318
replacing the card
Card-2
0001
File numbering is reset
File numbering after
creating a folder
Card-1
100 101
0051 0001