There’s a couple of things you need to think about:
- I would use hardware that runs VMware ESXi "out of the box". If you use any of the vendor servers like HP or Dell then you will be fine. If you build something yourself then you might run into issues with unsupported network drivers and such. You can make it work but it can be a pain.
- CPU power isn't much of an issue but memory might be, if you use the "small" image for the CSR1000V then it configures each router with 2.5GB of RAM.
- In reality each router probably doesn't consume more than ~1GB. I'd have to boot one to check if though.
I'm using a HP proliant DL360 G7 with 32GB of memory, these are cheap nowadays and support plenty of memory. Here's an example on Ebay:
Memory is cheap, harddisks are still pretty expensive but you don’t need much for some virtual routers. I also use mine to run some Linux / Windows virtual machines so I’ve added 4x 146GB drives.