If they are the focus of the painting, please see the above question.
If they are not the focus of the painting, they will be more "impressions" of people, with very little detail. This works for crowds or random figures in a shot. Again, if the figures are an important part of the picture (family members, etc.) sketching 2-3 in a small painting is probably not going to have the clarity and detail you are going for. Please just email me if you are unsure what size painting to go with!