No one on this site is qualified to answer your question. The only people who can answer the question in a way that covers you legally is Microsoft. No one else.
However, it is my opinion and understanding that for each installation of Exchange, you will a server license. With Exchange 2007 that will mean at least three server licenses required, as the Hub Transport and CAS roles cannot be on a CCR cluster.
Exchange CALs are per seat, so you would only need to purchase more of those if you changed version.
Simon.
