It depends on how you plan to use them. You need one unit 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 15 options or less, then a box of 15 is fine.
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 twice as many frames as participants is recommended, but your requirements could easily scale up to 4:1 with a creative and thoughtful group addressing a big open-ended question.