Are you asking about the Security Parameter Index (SPI) value? This is a uniquely generated value in the ESP header that identifies which tunnel (and secret key) encrypted VPN traffic is associated with; it is not a fixed value over a long period of time, only for the lifetime of a tunnel.
If you are using route-based VPNs employing a VPN Tunnel Interface (VTI), then you may be referring to the value displayed by fw ctl iflist:
[Expert@R8040:0]# fw ctl iflist
1 : eth0
2 : eth1
3 : vpnt1
Not sure if you are looking for the "ifn" value (which is 3 for the VTI vpnt1) or the "VPN Tunnel ID" which is specified when a VTI is created, and automatically becomes part of the interface name, "1" in this example for VTI vpnt1.
New 2-day Live "Max Power" Series Course Now Available:
"Gateway Performance Optimization R81.20" at maxpowerfirewalls.com