The upper part of the jaw is called the maxilla. and the lower part is called the mandible. You have 14 maxillary teeth and 16 mandible teeth. Perfect number. And, it is normal.
In general, wisdom teeth emerge between the ages of 17 - 25. In extreme cases, it can emerge at 16 or even in the 30s. When they erupt, people experience mild fever, jaw pain, tenderness, bleeding of the gum or difficulty opening the mouth. When they fully emerge, the mandible contains 16 teeth.
Generally, lower wisdom teeth erupt first. Likely your lower wisdom teeth have erupted giving a total of 16 mandible teeth, while the upper wisdom teeth have not yet erupted. Give it more time they will likely erupt and you will have 16 on top and 16 on the bottom.
If they do not appear until you are 21, There could be several reasons for this: the upper 3rd molars could be missing completely; they could be caught in such a way that they can't erupt (this is called impaction), or they could be present and just slow in erupting.
My advice is to wait until your 21st birthday and have your dentist take an x-ray of the upper third molar area. This will immediately show why this problem exists. Your dentist can then advise you what should be done if anything.
Hope this Helps