Is There a Way to List Writing Recommendation Letters on a CV?
I have not seen this and wouldn't recommend it, I would not consider writing recommendation letters to be under academic "service."

If you specifically mentored particular students then it makes sense to note that mentorship, but if they were just people who took your class then writing them recommendations is just part of your teaching responsibility, it doesn't mean anything for your qualifications if your students were successful when you wrote them recommendations, those are your students' accomplishments rather than yours.