Assist with calculation

[bbm8]

I am writing a short program to count the number of times a digit appears in a large number but I can’t think of what math I would use to do this. I have no code to post as I can’t start until I figure out the math.

[Larz60+]

convert number to string
get length of string

[bbm8]

convert number to string
get length of string

Thank you