Here are some guidelines to help you determine what your sleep number is:
Start keeping a journal in which you record the number of hours you slept, and what you were like the next day. Did you notice you felt relatively good? Were you rested, or did you still feel tired? Did you notice an increase in your irritability? How was your concentration? Keep track of this for about two weeks, at which time you should be able to see any patterns. For example, you might notice that when you get eight hours of sleep, you typically feel okay, but if you get more or less than that, your mood isn't as good and you're more irritable.
If you don't feel rested with the current amount of sleep you're getting, you'll want to try adjusting this amount to see if you can improve how you feel. Since the average sleep requirement is about eight hours, it will be helpful for you to work your way toward eight hours. This means that, if you're currently sleeping ten hours each night (and waking unrefreshed or feeling irritable), then you'll want to work on decreasing your sleep to eight hours. And, yes, this will likely mean you'll have to set an alarm! Reduce your sleep slowly -- by about fifteen minutes every couple of days -- and see how you feel at each sleep number (continue recording this in your journal). Similarly, if you're generally sleeping less than eight hours (and feeling fatigued or irritable during the day), you'll need to increase your sleep by going to bed earlier or getting up later, in the same way, by about fifteen minutes every couple of days.
When you get to a sleep number where you feel more rested and your mood is better, you'll have found your sleep number. Keep in mind that if you have problems finding it, there could be some health problems getting in the way, such as depression or sleep apnea, and you should consult your doctor.