20 is the default value of the maximum levels of nesting (in the web server's servlet container) you can have during request dispatches. If your JSP/servlet application is hitting this limit, you can increase it. See below for more information.
For Web Server 7.0:
Property name: dispatcher-max-depth
http://docs.sun.com/app/docs/doc/819-2629/6n4tgd200?a=view
http://docs.sun.com/app/docs/doc/819-3283/6n5h03i0j?a=view
For Web Server 6.1:
Property name: dispatcher-max-depth
Add the following as the first subelement of the <JAVA> element in config/server.xml:
<PROPERTY name="dispatcher-max-depth" value="35"/>