AS and A Level Economics: What is a good or preferred structure to follow for essays and is there a minimum number of points to include in answer?

Generally we would not want to prescribe a certain format and the structure of an answer does not have to follow a particular outline, i.e. an analytical point can be evaluated straight away or evaluation can follow the complete analysis.

Regarding the required number of points in an answer, it depends very much on the quality of the points. The evaluation should be a supported judgement based on the analysis and can include dependant factors or limitations.  

So the evaluation could be more a supported judgement of an advantage/a benefit or could also be reasoned to be more of a disadvantage/cost or 'it depends'. If a questions asks for ‘costs/benefits’ i.e. plural, more than one cost/benefit would need to be analysed.