How do we set an email alert once the connection on LB reached beyond some limit.. ex. 20k connection?

Hello Gajji.

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

REF - https://support.f5.com/csp/article/K45329113

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)

REF - https://clouddocs.f5.com/api/irules/ISTATS__incr.html

REF - https://support.f5.com/csp/article/K59616664

REF - https://support.f5.com/csp/article/K3727

REF - https://support.f5.com/csp/article/K3667

Hope this helps you.

Regards,

Dario.