Number of Messages in an Index Page

Is there a way to display 20 messages in an index page (thread or 
subject page), where each message is only the first one of a thread.
In my current app I want to have 20 first-thread-messages in an index, 
but unfortunately the messages following the first one of each thread 
increment the number. So if there are 18 replies to a message my index 
has only two messages which are at the top of a thread.
Harald