I'm going to asume your router also acts as a DHCP - for this reason as well as a few others only 1 router. Or very bad things will happen.
On a network of that size I'd have - 1 router (connected to internet as needed) with a gigabit interface to a hub using a gigabit port - then the hub to each PC using 10/100 or 10/100/1000 (gigabit) depending on budget and expeted network usage
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