Grandstream also makes some 16/24/32/48 port FXO gateways. We use their smaller 4 and 8 port models with AsteriskNOW/FreePBX and they work just fine.
http://www.grandstream.com/index.php/products/ip-voice-telephony/enterprise-analog-gateways/gxw42xx
As stonet points out, though… not cheap.
Eric / www.rkt-tech.com