SUPPLY SIDE: "send him to study [x], it as at least twenty to one if he ever makes such proficiency as will enable him to live by the business." He is making is quite clear that the supply of qualified [x] is low (and giving a reason as to why it is low, but as Adam Smith himself would surely admit, the laws of supply and demand do not [directly] care about the 'why').
DEMAND SIDE: There is no explicit mention of the demand for these services, but as educated readers, we are able to deduce that during the time of Adam Smith, the demand for certain [x] heavily outweighed the supply.
*[x] blocked out because its specific value is not germane to the underlying logic of the argument.