22000k Users / 10G Firewall/SWG

Does anybody have a feeling what FortiGate model could be good for a 15-25 k users network. Will probably be a central perimeter firewall dividing internet from wan/mpls to inspect internet traffic. Mostly basic utm maybe even with TLS Interception or explicit proxy. I’ve not gathered much info yet to be honest.

As far as planned itll be set up with 10G internet access.

I have a pretty good idea how to size everything to 500 users oder so (mostly nothing bigger than 100F or 200F), but here I’m a little overwhelmed.

I’m trying to calculate with 10-15 sessions per device/user. I feel like 400F is more than sufficient if tls interception is not needed.