the plugin doesn't have any limit per se, but you have definitely to refer to the limit the server is currently hosting your website has.
For example, all my domains are on Justhost.com and they have 75 emails every 30 minutes limit. So if you are sending single emails to separate user you cannot send more than 75 in 30 minutes. Once you reach that limit, all the emails sent during the following 30 minutes will fail. But pay attention to this: if you are sending a single email with more recipients than the number allowed by your server that email will fail and no one will be sent.
I am working on a major upgrade (possibly version 3.0) that will also help manage that kind of situation.