We purchased 2 appliances 5500 and begin implementation. So few questions:
How many users or simultaneous requests can support one device (ssl, filtering and both antivirus engines is on)?
If our load is more than this appliances can support - we need to purchase уet such device or we can purchase virtual or image for install on our servers?
I'd suggest to contact your sales rep or SE in order to work with them on getting this information. There are so many factors surrounding sizing and performance that giving even a rough figure might be wrong.
MWG performance is measured in Request/Second, not in simultaneous connections, number of users or bandwidth. That's why questions like that are not really easier to calculate. For example, 1000 Requests/sec could be anywhere between 20,000 to 70,000 users or 80MBit/sec.
The only way to be sure is to measure current activity from existing logs to properly size the environment.