If you just want a count, you can use the script that is there for RRD:
/usr/local/bin/php-cgi -q /usr/local/bin/captiveportal_gather_stats.php '<zone name>' 'loggedin'
/usr/local/bin/php-cgi -q /usr/local/bin/captiveportal_gather_stats.php '<zone name>' 'concurrent'
