When you invoke the Generate Serial Numbers Dialog, after you have chosen the quantity to generate, the Authorization Definition to generate for, and entered any required parameter value, you must enter the number of Permitted Activations and Overflow Activations (if any).  This is shown in the following dialog:

 

Above image shows entering a values that will allow 1 activation per Serial Number and 3 Overflow Activations (effectively 4 activations).

Once you have completed entering the number of Activations, click "Next >>" to continue.