You use 3 cat5 cables per speaker. So take 3 5m lengths (maybe longer as plaiting can shorten the finished product depending on the tightness) for each cable you want to make. You will probably need about 35m for a 5m pair.
The FFRC recipe is flawed in my opinion (shared by others in that it's easy to get wrong. CJROSS came up with the CC-FFRC which is easier. See here for similar questions and answers to yours.