Is comment a mass noun or countable noun>
The teacher wrote some comments on their workbooks
The teacher wrote a comment on their workbooks
'comment' is a countable noun.
Are you sure you don't mean, 'in their workbooks'? If the teacher wrote the comment(s) on their workbooks, it suggests it was written on the front cover of the book, on the outside of the book, not inside the book next to each assignment they had completed.