Very much bandwidth permitting you can archive remote sites to a single EV server and I know of numerous companies that do this.
I would personally probably put them all under a single site if I had an EV server at each location. You then only have one EV Directory SQL database, opposed to what would be multiple otherwise as they would each be completely seperate. I guess it could come down to costs etc.
There is a EV Planning Guide availble for planning deployment of Enterprise Vault. This is available from the normal support location and selecting documentation in theright hand side.