almost 4 years ago - SylenThunder - Direct link
Max supported player count is 8 players.
Max stable limit on average server-grade hardware is going to be 20 players.
Max stable limit on extremely high-end server hardware is going to be 30 players.

OP's hardware is towards the lower-end. It's capable, but with the lower clock speeds they're going to struggle. I would put the bottom-end at a 3-3.5Ghz system with a minimum of 6 cores.
Another consideration is the drives. Most of those older systems are going to come with a pair of SAS drives, usually 10,000 RPM, but some higher. Those aren't going to be quite fast enough for supporting more than about 6 players until the game is optimized more. You will want to add in the cost of purchasing some SSD's for running the servers save data on.