I need to install CUCM to serve 3.500 users, is it possible to install it on a server IBM x3650 2 quad core, 4GB RAM, 73GB HD, 4 ethernet?
You will need to identify what CUCM version you are going to build
as a first step. Then move forward with picking a server model
This is the "master" list for Server support.
Supported Cisco Unified Communications Manager Releases by Server
Hope this helps!