Dear @wamendoza,
This is a complex question and depends on many factors (your topology, environment, the scope of the use, etc).
You can check the FortiGate model's datasheet to see the values for each version and decide for yourself.
https://www.fortinet.com/content/dam/fortinet/assets/data-sheets/Fortinet_Product_Matrix.pdf
https://www.fortinet.com/de/resources/datasheets
Another way is you need to contact your Sales Engineer. They can take in your request Systems Engineer (SE) that covers your territory.
http://www.fortinet.com/aboutus/locations.html
Alternatively, it can be done through Regional Sales Partner Channel
http://www.fortinet.com/partners/reseller_locator/locator.html
Also, you can get in touch with our Professional Service Team, they will help you with the whole process.
https://www.fortinet.com/support/support-services/professional-services
Best Regards,
Vasil