Crunches focus on one specific action - spinal flexion. The "core" as a unit is designed to stabilize and support our spine. The core is made up of all the muscles that would fall under these categories: low back, abs, and hips. Additionally, these include both the superficial and intrinsic muscles - there is a LOT going on in this part of the body.

For years, crunches were the "go-to" exercise to tone the mid-section. Nowadays, it's becoming one of the least functional exercises available and what most people's core really craves is stability. The plank is one of the most popular exercises but there are many others.

Don't get me wrong - the crunch does have it's place. But the average person needs to work on stability and their ability to resist rotation much more than doing crunches. In efforts not to stray too far from your original question - I would say the number depends on why you are doing in them in the first place. Generally speaking moderate rep ranges apply for all strength exercises, if you are looking to improve your crunch endurance - do more reps, if you are looking to build strength in abs - I would do less and experiment with external loading.