Since version 11.5, Plesk does not place restrictions on the number of email addresses listed in your whitelist and blacklists.
By default, Plesk allows you to have 100 email addresses listed in your whitelist and blacklists. This decision was made by Parallels, the creators of the Plesk Control Panel, to help reduce CPU usage caused by SpamAssassin when dealing with large lists.
Since Plesk Control Panel version 8.4.0, you may configure maximum number of addresses that can be added to the SpamAssassin While or Black lists. Please see this Parallels article: kb.parallels.com/en/5640.