These start to get a bit pricey, so if you want one, check Amazon.com for renewed or amazon warehouse deals, eBay.com for open box with a manufacturer's warranty, and the Behringer Refurbished Factory Store for the best prices. And don't overlook extended warranties. Be careful with used equipment sales of these items, more than likely they're out of warranty and could have hidden issues that can be costly to repair.

4 MICROPHONES
Behringer XR12 12 input

8 MICROPHONES
Behringer XR16 16 input, 2 channel Digital USB output

16 MICROPHONES (the one I've been using for the Jazz on J Street Jams, just so you know, I got mine for around $450, as a open box with warranty on eBay and it was well worth the wait)
Behringer XR18 18 input, 2-18 channel Digital USB output