What is the cylinder numbers on a 2000 silverado 4.3 motor?

The 4.3L Vortec V6 engine in a 2000 Silverado is not numbered like a V8. V6 engines have 1, 3, and 5 on one side of the engine and 2, 4, and 6 on the other. There's no standard "cylinder 1" location like on a V8. The firing order is what's important for things like spark plug replacement and diagnostics. You'll need to consult your owner's manual or a repair manual specific to that engine for the firing order.