This assessment is based on the area to be transplanted and the eventual hair density required. In most case we keep hair density of 35-45FU/cm2. A rough estimate is given below (inred) in terms of follicular units. Kindly note each FU contains 2-3 hairs.
NOTE: This is just a rough estimate, actual requirement in your case might vary from this. Kindly consult your doctor for final estimate.