How to limit concurrencies
You can limit the number of concurrencies on web studies by setting the html_limit in the parmfile.
HTML_LIMIT=X (Where X is less than the your license)
It will stop letting sessions in at that value if it is less than your license. Typically though you only need to use the html_limit option if you are running multiple study servers or your license is set higher than the machine itself can handle.