Can we have multiple tunnels on a router, all of them having same source IP address or same source interface?
I see what you're trying to do and you can do it that way.
Although, that is the old way of doing things.
You're probably better off using a DMVPN solution.
In that case, the hub router will have one multipoint GRE (mGRE) tunnel interface and its IP address will be on the same subnet as the tunnel interfaces for all the spokes.
You will use NHRP to have the spoke routers (NHRP clients) advertise their outside IP interface address and map it to their tunnel interface.
The hub router (NHRP server) will build a database of all the spoke's IP address-to-GRE tunnel-address mappings, which allows spokes to be added on dynamically and with only one mGRE tunnel interface at the hub supporting all the connections.
This is the general approach. You run GRE over IPSec and utilize NHRP for dynamic additions to the DMVPN network.
[EDIT] We can pick this up tomorrow, if you need more info, or a sample config. Right now, I need to crash. [EDIT]