If you're reffering to entires in the SiteLog table:
You can set the type of events logged and the number of days the log is kept but not the number of log entries.  If you need to cap it at 500 you'll need to either add a trigger to the table to purge excess data (will always keep it from going over 500 but adds to core DB config/schema) or add a scheduled script or SP to periodically purge data (will not prevent log from going over 500 but has less "footprint" on core DB config/schema).
 
Hope that helps-
-mamlin

There are no built-in features in DNN for truncating the Event log as far as I know.  You can schedule jobs on your SQL server to do the trick but not everyone has that kind of access (or knowledge) to do so.  Fortunately you can easily (and freely) use a "SQL scheduler" module to execute your own command on a regular basis (once every day?) to truncate the log down to the past xx days.

Here's a free scheduler module (must have HOST account access for your DNN installation):
 
   http://www.iowacomputergurus.com/free-products/dotnetnuke-modules/scheduled-sql-jobs.aspx
 
 
And here's a simple SQL command to truncate your EVENTLOG table down to events from the past 50 days (warning -- try this on a test copy of DNN first, not on your production copy!):
 
   Delete FROM  EventLog
   WHERE (LogCreateDate < { fn NOW() } - 50)

 
Cheers!
-mamlin