Status > Monitoring
click the wrench, Left Axis: System, States. Update graph.
Change the duration of the graph and look at the highest resolution available to get the most accurate picture.
You can also look at CPU utilization, Memory, etc there.
A good view of current conditions is Status > Dashboard
. For connections look at the state table size. But for system sizing I think the historical graphs are more valuable.
My guess is you won't be close to the capabilities of the SG-2220
and the real decision is how many physical interfaces you need. For HA/CARP I would set the minimum unit at an SG-2440
. WAN, LAN, SYNC, and an available OPT.