Not all the space that you get from extractions will be used to retract teeth. If you had crowding, unraveling the crowding and straightening the remaining teeth will take up quite a fair bit of the space from extractions. The remaining space, depending on whether your teeth are protruded or not, will either be retracted, or your back teeth may be moved forward to close the gaps.

These decisions are made when your ortho plans your treatment based on the calculations your ortho did from your study moulds, xrays, photos, etc. I suggest voicing your concerns to your ortho. He/she would be able to better explain these things to you based on your specific case.