This is a very open ended question and is very much dependant on your hardware, mail server and restrictions set outside the scope of the program. In theory, the current maximum number of e-mail addresses that ListMessenger can handle is the maximum number of MySQL inserts that your server can do within the set maximum execution time. There are many different ways to benchmark this number, however, I can give some real life examples.
The maximum known number of subscribers is with a company that supplies fire and emergency services departments with their tools of the trade. Their most recent number of users was over 950,000. I know they have powerful server farm running their setup; however, with little to no modifications they have successfully sent out several mailings.
There are other users that have mentioned to me that they have a large subscriber base and haven't had any troubles sending 80,000 or 90,000 messages out.
While these are large scale examples, there are many websites using ListMessenger to send out to smaller subscriber bases such as 50 - 150 users and beyond.
Please keep in mind, ListMessenger is not a Spam tool and I take this very seriously. There are advanced tracking systems built into the program that make it easy for me to identify the individual and reseller behind any spam messages. Not only will your account and keys be black listed; I will sleep better knowing that a) Your personal information was posted on Spamcop b) I have reported you to every authority that would do anything about your Spam :)