It depends on how you plan to use them. You need one for each idea or option that you want participants to rate (score-vote).
If you are using them to vote on a predetermined set of options, then 15 is generally fine.
But if you are planning to invite participants to generate ideas for rating, then 15 units can quickly feel very limiting among a creative group of people. Trying to combine or shortlist ideas in the middle of a workshop, just to fit within a limited set Feedback Frames, can be frustrating for participants and facilitators. Having a minimum of 1.5 frames for every participant is recommended, but your requirements could easily scale up to 4:1 with a creative and thoughtful group addressing a big open-ended question.