You can put several shapes in one sheet: the shapes you create or any other shape or object "belonging" to other sheets.
A simple sheet file may look something as this:
<?xml version="1.0" encoding="utf-8"?>
<sheet xmlns="http://www.lysator.liu.se/~alla/dia/dia-sheet-ns">
<name>Circuit with identifiers</name>
<name xml:lang="es">Circuito con identificadores</name>
<description>Components for circuit diagrams</description>
<description xml:lang="es">Componentes para diagramas de circuitos</description>
<contents>
<object name="Circuit with identifiers - NPN Transistor">
<description>A bipolar npn transistor</description>
<description xml:lang="es">Un transistor bipolar npn identificable</description>
</object>
<object name="UML - Objet">
<description>An UML object</description>
<description xml:lang="es">Un objeto UML</description>
</object>
</contents>
</sheet>