Rotor comes with and should have 21 knives on it like the book says. There is provision to add 21 more for a total of 42, but unless you're in rice or perhaps a tough rye, stay with 21. The more knives you run, the more straw will be broken up, but the smoother the rotor will separate. 9 or 12 is a good number for dropping straw, but if you're just chopping or spreading it, put in all 21. The ability to add or remove the rotor knives is just one thing that makes this combine so versatile and easy to set.