At this point I am running anywhere from 3-8 VMs at any given time depending on what I am testing out. It is a mix of Linux and Microsoft Operating Systems, along with a few 'turn key' virtual appliances (Security Onion, Web Security Dojo, and more).
Here is a breakdown of the hardware I am using:
AMD FX-8150 (8-core)
4x 8GB Corsair Vengeance LP (always been a fan)
3x 250GB Western Digital (random drives I had around)
1x 1TB Seagate Barracuda ST1000DM003
Cheapest Rosewill case for mATX, Corsair CX430
Syba SY-PEX24028 PCI-Express Dual Gigabit NIC
I am using one of the ports on the Syba NIC configured in promiscuous mode to monitor network traffic sent by a 'Mirror' port on a TP-Link TL-SG108E 8-port switch.
My implementation of ESXi is basic, no frills. I wanted something where I could spin up an OS and roll with it. And this works perfect for me.