You can install Enterprise Manager on the same server as either VBR or Orchestrator.  Typically, if you are installing it then most use the VBR server.  It is up to you if you want a separate server or not.

I prefer one server per application/role.  But honestly, EM can coexist just fine with VBR.  I’d probably not put it on the Orchestrator server personally.  Also depends on if they exist in different locations.  Often I see VBR in the production datacenter and obviously Orchestrator should exist at the recovery site.  Personally, I’d recommend all three servers or at least all three roles reside in the recovery site.

EM isn’t too bad to have on the Veeam server, but I personally created a new server for it. If and when things grow or if I decide to upgrade or replace one of them it’s just less to worry about. 

It also comes down to security. Enterprise manager can be accessed by a webpage, permissions given for reporting, labs etc. The people that access that don’t have access to my Veeam server.  My database admin can restore his DB from Enterprise manager, but doesn’t need direct access to the Veeam server either.

As far as resources, enterprise manager usually doesn’t use much, but it sure is a handy tool to have.