Social Alerter will allow you track an unlimited number of sites. However, if we feel a user is burdening the system, we might contact you to try to sort something out.
So... track away :)
Some people are wondering what's the catch with offering the Social Alerter for free. The short answer: there is no catch. The long answer...
This post describes how Social Alerter (SA) works in detail...
SA tracks in real-time social bookmarking and social news websites (the list is here). While watching these websites, it looks for the websites you've specified for alerts. If it finds that a story is getting hot, it sends you an email warning you about the likely flood of traffic. This allows you to prepare your site before it crashes under a lot of traffic.