I found out I have a mild shift to my knee cap and torn ligaments and cartlig missing. Would I need a cast after surgrey?

Restrictive bracing . Very rarely (to never) is the knee joint actually casted. In situations where a knee should not be flexed following surgery an immobilizer or brace (locked in extension) may be applied. However, these options are more conducive to post-op pain management modalities, bathing, and monitoring of incisions for infection. Most likely you will only have to deal with a brace and crutches.
Usually not. After ligment realignment surgery, motion and therapy are important. While you may need a brace, usually you are allowed motion in certain planes to help with your rehabilitation.