I want to install multiple OpenVPN instance on one machine(Centos), and it seems that each instance needs a unique subnet, and how to do this? And how to install multiple tap/tun
The subnet part is pretty straightforward, your "--server" directive lists the subnet involved:
There are a bunch of things you need to do when running multiple instances, see this page:
(search for "multiple OpenVPN instances")
I don't think you have to do anything special to create the tun1 device I think OpenVPN does it for you on CentOS but I'm not sure about that.