Re: How to choose the Fortigate Model that fit your needs:
First, forget about the number of users in your network. There can be networks of 10 users that may need more throughput than networks with 100 users. Also, considering number of sessions, sessions/sec and such metrics are pointless, as nobody can estimate such things in advance.
What I'm suggesting is the following:
1. Define your needs first, in terms of what security features and profiles you must deploy in your network and where.
2. Define your data flows per traffic direction: north-south & east-west
3. Define your throughput needs per traffic direction (that's the most difficult)
4. Define other needs, like IPsec/SSL needs, VDOMs, access points, fortiswitches, etc.
5. Define your future needs.
In the north-south direction, the actual barrier is usually your Internet speed. This is normally much lower than the threat protection throughput of your firewall. The east-south direction is more difficult to estimate in advance - it depends also in the nature of your network. DCs have normally huge east-west traffic needs.
After you have done all this housekeeping, try to estimate your aggregate worst case throughput needs, which is the threat protection throughput, having in mind that the performance numbers provided by Fortinet are referring to non-encrypted needs, while the majority of the traffic nowadays is encrypted. Therefore, the figure that would be closer to the aggregate throughput needs should be SSL inspection throughput. Finally, allow a minimum tolerance of +50% in that figure and you are done.