Variable, if you have weights you can do weighted crunches. They're strenuous, so you need to do less of them to feel the same burn in a shorter amount of time. Plus it helps guys get six packs easier.
The most simple way of doing a weighted crunch is with a dumbbell or a small weight ball. You lay on the floor with your hands straight on the ground above your head, like you're reaching for something. Stretch them out as far as possible, holding the weight between them. Then slowly focus on your abdomen for strength to come up to a sitting position (or even further.. almost hugging your knees) bringing the weight up from above your head into the air and back down in front of your feet. Kind of get the idea? It's kind of like the following picture, except you're not on that 'chair' thing. Plus, you can either keep your knees flat or as shown in the picture.
If you do have access to that bench thing, go for it. Most gyms have one. But be careful. You need crazy back support/strength/flexibility. So you need to be working out a while. It's mostly also about placing the right amount of pressure while doing the move on the right parts of your body. And usually just working out regularly helps you get in tune with yourself like that. It took me a few months to build up the courage to get on that thing. Works well though. I don't use weight while on it yet. Just work against my own body weight to pull up.
Also some invaluable tips I've compiled from experience (I think I wrote a long essay on the old Islamica once..sigh)
1) DO NOT strain your neck while doing crunches. If you feel the slightest bit of strain on your neck, you're doing the move wrong and should stop ASAP. It means you're using your neck to crunch up, which isn't even a crunch really. Your abs are playing close to no part in this move. And of course, you can injure your neck pretty badly. Oh and people who do crunches correctly can also start feeling neck strain as they get closer to the end of a set of crunches. That just means they need to end the set and rest for a few. To avoid straining your neck, touch the top of the inside of your mouth with the tip of your tongue. It helps.
2) Your fingertips should be like on the base of your skull, right above your neck when doing crunches. Both your elbows together should form a straight line when you do so or you shouldn't be able to see your elbows during a crunch (except from the corner of your eye). Look straight up at the ceiling at all times, it helps with proper crunch posture.
3) Make sure you crunch slowly and if possible close your eyes and take all your energy and focus on the body part you are working out - in this case that would be the abs. Just visualize your abs while thinking of taking all your energy and putting it into each crunch. I know it sounds weird but it really, really helps with proper posture. Often closing your eyes really really helps because the visual distractions are gone and you can kind of channel the 'sight' energy into the body part you're working.
4) Suck your stomach muscle in with each move. When you feel like the middle of your stomach is being pulled forcefully to the ground right underneath your spine, you know you're doing it right.
5. Every time you put in all your energy into a move, your breathe OUT loudly. So when you come up to crunch, you exhale. Proper breathing helps you to utilize max potential/endurance. Often people have more endurance than they can even think of; they just don't know how to breathe right (with cardio or strength training). Don't hold your breath; people do that without knowing because they're stressed out about lasting until the end of their reps.