Weight Lifting Repetitions
There are many different reasons why people strength train (i.e. lift weights). So, it depends on what the goal is for strength training. Generally it is safest to lift weights slowly, using a weight that can be repeated 10-15 times. The speed at which you lift the weight should be 2 to 4 seconds. Lifting the weight in a 2-second count and lowering it in a 4-second count.
However, if you are an athlete performing explosive-type movements, or speed and agility-type exercises, you would change your workout entirely to be able to meet the specific challenges of the sport.
The same rules apply for crunches. Your abdominals should be trained the same way as any other muscle group. Slow and controlled movements are the safest way to train your abdominals and core muscles.