I'm a little over 21 weeks and my doctor told me that at this point it wasn't necesssary to keep track of how often the baby moves becuase like Niki said, you won't feel all the movements. He said starting at 25 weeks is when we'll start the "kick count".
Hope that helps! :)
Kicks are still pretty inconsistent at 21 weeks. They'll become more consistent between week 24-26 (when you should feel them everyday).
I don't think that there are any "rules" about movement at 21 weeks. At that point the baby is still pretty little and you won't feel all of the movements. I know that at around 28 weeks most Dr's will recommend that you start "kick counts" where you count at the same time (babies most active time) every day and at that point you should feel 10 movements within 2 hours.
Ofcourse if you are ever concerned about sudden lack of movement you should put a call into your practitioner.