F-SECURE ANTI-VIRUS FOR MICROSOFT EXCHANGE 8.00 Administrator's Manual page 245

Number of spam Specify the number of Spam Scanner instances
scanner instances to be created and used for spam analysis. As
one instance of the spam scanner is capable of
processing one mail message at a time, this
setting defines how many messages undergo
the spam analysis simultaneously.
You have to restart the Content Scanner Server
after you change this setting to take the new
setting into use.
IMPORTANT: Spam analysis is a
processor-intensive operation and each spam
scanner instance takes approximately 25MB of
memory (process fsavsd.exe). Do not increase
the number of instances unless the product is
running on a powerful computer.
CHAPTER 6
Administration with Web Console
245