You must analyze which objective instances are required for each case. Typically the creation of objectives falls into one of these patterns:
For a given objective type, there is always exactly one objective instance for the case; and/or
For a given objective type, there is one objective instance for each member of the case who meets certain criteria.
For example, a product aimed at a household with children might always deliver exactly one "basic household allowance" objective to the overall household, but also deliver a "child allowance" objective to each child. Thus a household with 3 children would be have 4 objectives created - one for the household and one for each child. A household with no children would have only a "basic household allowance" objective created.
Note that the Engine does not impose these single/multiple objective patterns; the creation of objectives can be as complex as your business requirements dictate.