Plural of feedback
Feedback, as you acknowledge, is not a countable noun. Therefore, to indicate plurality it is necessary to attach it to something that does have a plural form.
- Feedback from 14 sources (or respondents, participants, etc.
- Fourteen feedback messages
- Fourteen instances of feedback
I do see some indications online (blogposts and the like) that feedback may be considered by some to be a countable noun now and that a plural form, feedbacks, may be used. However, it seems awkward to my eye and ear--no doubt due to unfamiliarity.