Fortigate 500D - WAN Ports

The 500D has 8x GE RJ45 ports and 8x GE SFP slots.  I believe you can use any of them as WAN ports as they're not restricted, thought that will of course reduce your number of ports to use for LAN.  And you would need tranceivers for the SFP slots.  See https://www.fortinet.com/content/dam/fortinet/assets/data-sheets/FortiGate_500D.pdf for details.

Note that Fortinet has published the details on their new 500E, which includes adds 2x 10GbE SFP+ slots.  I don't know about availability (some sites are saying it ships in 6-8 days).  As usual with brand new hardware or software I would wait and see how stable it is before buying one.