100 works well like said above. Because miners are chatty, you just need good gear that can deal with a ton of packets per second. This is because when miners submit packets of data, they barely fill each one as they are sent out.

Search on ebay for used Cisco switches, as older model enterprise gear is perfect for mining

I HIGHLY recommend the Cisco Catalyst WS-C3560G-48PS-E Switch

Also note that as you expand your operation, you will need to be sure you have an adequate router as well