Many customers send 2-3 CD’s each month, with each box. However, if you have a large quantity, this likely will not work for you. It is permissible to remove the foam and fill the box with CD’s, in which case it will hold about 15 CD’s in their cases. (Include a note to let us know if you need the foam insert or you kept yours and will reuse it).
In the big picture, CD’s are getting harder and harder to recycle. The plastic markets are down, and they are less and less valuable. Before sending them in, check if your local Goodwill, or record store, wants any of them.