i want to generate Trusted SSL Certificate for my web server to be installed on F5 .
as i understand i will generate CSR from F5 , i have 2 boxes in HA , will i need 2 certificates one for each ...
You'll only need one certificate, it'll be passed between devices by ConfigSync. Strictly speaking you should tell your CA you'll be using the certificate on two devices (and probably pay more) but most don't I'm sure.