Feedback Frames only come in boxes of 8 units (A4 size for international customers), and 15 units (US Legal size for US and Canadian customers).
How many units you need 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).
The maximum number of tokens that fit per column is 24 in the A4 size unit, and 30 in the US Legal size units. Assuming everyone will not vote in the same column, you can plan for about 30 (A4 size) to 40 (US Legal size) votes per frame.  If your group is larger than that, you can either accept a sample of 30-40 responses as good enough, or you can duplicate ideas across multiple units so that everyone gets a chance to vote for the same options.
If you are using them to vote on a predetermined set of options, then it is easy to plan your required quantity of frames.
If you are planning to invite participants to generate ideas for rating (like a brainstorm), then just 8 or 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. It's better to have too many units than too few.