Two ways to get it.
1) Use an external monitoring system (let say Zabbix, Cacti, etc) to poll connection values of one or more VS using SNMP
2) Use an iRule to increase/decrease an iStat variable (to obtain the current connections). When the istat value will be above some threshold, you could trigger an alert using iCalls or by logging a message (with customer alerts)
Hope this helps you.