How often is paralysis certain after spinal cord injuries?

Variable. Kind of the luck of the draw. Sometimes the cord can be bruised with near complete recovery. There are multiple factors: level of the injury, violence of the injury, etc. Impossible to predict in advance.
Not certain. Not all spinal cord injuries result in complete paralysis. There is often weakness and sensory loss. The bowel and bladder can also be affected. The level of weakness is related to how profound the spinal cord injury is.