Just as many locks as you have places to fit them. If a thief looks at a bike with like a foot of hardened steel he has to cut through he's just gonna say fuck it and look for a scooter. Also varying the kind of lock is a good idea too. It's like rock paper scissors, you know, hack saw beats cable, cable beats car jack, car jack beats U-lock, bolt cutter beats chain, chain beats etc.etc.etc. So the more tools and shit he has to carry around, chances are there's gonna be one he can't get through, also each lock increases the amount of time he's gonna spend at it, and increase your window of time to shoot him and put him in a wheel chair for life, where he belongs.