Believe it or not SBS cannot count properly. The licensing service has often been a problem, so much so that it has been completely removed from SBS 2008.
However if it says 5 CAL's you need to install or re-install the other CAL's. You are likely running out of licenses. SBS has 5 "ghost" licenses, such that if there are 5 CAL's installed you will not have a problem until the 11th device or user connects. SBS comes with 5 CAL's pre-installed, so it sounds like you have not installed any additional licenses.