1) 114 Domains / 524 mailboxes on a (heavily tweaked) vhcs2
2) 225 Domains / 1600 mailboxes on an ispcp RC6 (recompiled apache to increase the max descriptors).
3) 52 Domains / 422 mailbox on an ispcp 1.0
All of them providing all services themselves, including amavis+clamav+spamassassin mail filtering.
After the ispcp-apache-logger and
enhanced logfile handling, I think the next major "showstopper" as to the number of domains is fastcgi itself consuming all the available ram. Maybe this could be alleviated by allowing to setup the min/max number of workers through the gui....