How to know how many characters there are in a text file

If you want to know, how many printable characters (letters, number etc.) in the file, you'll need to count them.
If you want to know quantity of all characters (including spaces, new lines etc.) in the file, simple get file size in bytes if the file is ASCII file and half of file size if the file uses wild characters.
Finally, if you want to know roughly how many printable characters in the file, you can count an average percentage of printable characters in some common text files and calculate the roughly value using that percentage and file size.

Jack.