for me it would be in the multi-rename tool nice for having the number of digits automatically adapted to the number of files which I want to rename. In this case automatically be changed / by default to "'2' digits":
Or as compromise: using "#" as placeholder in the rename field as alternative to the »[C]«. In the above example: »##_new_name« = »[C]_new_name« = »01_new_name« ... »10_new_name« (like ACDSee).
I'm sure that ## won't be added as a replacement for [C] because ## is a valid filename part.
But you can use [C:2] for 2-digit number... and even [C:a] for exact thing what you've asked for - for autodetecting of number of digits. Just press F1 in MRT window to see all available options.