When you initially flow all of the portraits onto pages using Portrait Auto Flow you will select how many portraits you want per page. Later you may want/need to change the page layout for all or some of the pages.
1. Go to the page where a new layout is needed and select Portraits.
2. In Edit Portraits, select Photos Per Page. If you want all the portraits smaller to use less total pages, the photos per page should be a high number. If you want larger portraits, use a lower number.
3. When you select a photos per page number you will see how many pages will be needed for the portraits. Here is the page with 36 portraits instead of 30.