We do a similar method to what Dwight has mentioned. I set up a sql job that collects the user data and open screen counts on each of our company databases and dumps it into another db/table. Then I use SSRS to display usage graphs on our tech support Sharepoint site.
Looking at the graphs, we can identify peak times and user load as it spreads across the different company DBs (which we also use for diverting license cost to the correct place).
Also, you could likely use a similar method that could email mail a warning when your user count gets to a certain threshold if you wanted a more automated notification approach.