I am using tomcat 5.5 for serving more that 100 sites(suppose my-gwt-site-1.com to my-gwt-site-100.com).
All my sites are utilizing just one demonstration of a formerly used gwt application, the only real deference together is within their "Host" configuration section in server.xml :

<Host name="my-gwt-site-1.com" appBase="webapps">
<Context path="" reloadable="true" docBase="/my-gwt-app/" debug="1"
unpackWARs="false" autoDeploy="false" xmlValidation="false" xmlNamespaceAware="false" crossContext="true">

<Resource name="jdbc/mysql/db_1" auth="Container" type="javax.sql.DataSource"

initialSize="2" maxActive="50" maxIdle="5" 
maxWait="15000" removeAbandoned="true" removeAbandonedTimeout="2"
validationQuery="select now();"

username="root" password="pass" driverClassName="com.mysql.jdbc.Driver"
url="jdbc:mysql://localhost:3306/db_1"
/>
</Context>

&lifier it repats fot all my sites...

<Host name="my-gwt-site-100.com" appBase="webapps">
<Context path="" reloadable="true" docBase="/my-gwt-app/" debug="1"
unpackWARs="false" autoDeploy="false" xmlValidation="false" xmlNamespaceAware="false" crossContext="true">

<Resource name="jdbc/mysql/db_100" auth="Container" type="javax.sql.DataSource"

initialSize="2" maxActive="50" maxIdle="5" 
maxWait="15000" removeAbandoned="true" removeAbandonedTimeout="2"
validationQuery="select now();"

username="root" password="pass" driverClassName="com.mysql.jdbc.Driver"
url="jdbc:mysql://localhost:3306/db_100"
/>
</Context>

the only real reason I actually do the reason being my application must make use of the separate resource for each domain.
when number "host" sections in server.xml is all about 5, the isn't any problem, however when it matures to around 10 or even more, the tomcat server begins effectively however it cant load any one of my website(appears it crashes without anyone's knowledge quietly!)
I wish to know is a great choice to include lots of "host" tags in server.xml or there's much better method of doing this? Attention that Among the finest to make use of one code with separate databases its my sites.
Another real question is that the number of java sites(each with about 500 customers and low traffic) typically be run on one server(with 2 Quand Core Processor chips &lifier 4GB of RAM)?