The Patton Smartnode SN4932 will support up to 32 FXS ports.
http://www.patton.com/voip-router/sn4900/
Finding the proper template is the trick. You might want to contact Patton directly.
http://www.patton.com/resources/index.asp?a=12&
I'm sure that there are other larger ATA units as well. It is a matter of finding a supported device, or a manufacturer that works closely with 3CX and can supply a template for provisioning.
there are probably are other forum members that have been involved in a large hotel installations and could provide some insight. Analogue sets are still quite common in that environment given the durability, and if they wish to re-use existing sets, or re-wiring for SIP sets is out of the question.