Ann: Afterpay Share Purchase Plan, page-40

It's worth applying for the maximum. Any excess over the $15,000 sent, gets returned to the applicant.
There is sometimes a minimum given to let's say someone who would be scaled back to less than "x" number of shares because it is too administratively painful. They will get the minimum and beyond the minimum then it will be proportionately. From memory, I can recall such arrangements for deals such as this one in other companies.