AppleScript: How to get the character count of an .rtf file?
You asked, "What is the best way to go about this?" and while I'll not make that determination, nonetheless, here's an example using the do shell script command and some command line utilities.

set theRichTextFile to quoted form of "/path/to/filename.rtf"
set theCharacterCount to do shell script "textutil -stdout -convert txt " &amp; theRichTextFile &amp; " | LANG=en_US.UTF-8 wc -m | sed 's/ //g'"


Note that theRichTextFile must be a quoted POSIX pathname.

Update: Per comment by @jackjr300, added LANG=en_US.UTF-8 to the command.