For the sake of the uninitiated, “STU” = Standard Typographical Unit. It is a term infrequently used by educational institutions to specify the number of characters (letters, numbers, punctuation marks, spaces, etc.) per page. The reason for the infrequent use is likely due to the fact that this metric is essentially a factor of a host of different attributes, for example:
- page size;
- page margin;
- font (encoded metrics);
- paragraph style (indent, spacing, alignment, etc.)
- character style (effect, position, etc.)
- advanced typographic features (ligatures, kerning, etc.)
To answer the question, there is no such facility within Writer that I am aware of.