If a form has a fixed layout, the number of pages in the form is also fixed. If the form contains subforms that adjust to accommodate the amount of data being merged, LiveCycle Forms ES adds additional pages automatically if the amount of data to be merged cannot completely fit on a single page.
Note:
When you create a form design for a form that has a fixed layout that accepts merged data, you must ensure that filled objects do not expand to the point that they overrun the content area. Otherwise, unwanted pages may get added to the form automatically when the form is rendered.
A form design may contain more than one content area. For example, a master page may have two content areas to provide a two-column layout, and two master pages of this type (one odd and one even) may be needed to support double-sided pages.
See also 

When to add a master page