A. ADFS works well with trusts, so the answer depends on the trusts between the domains. Essentially, the ADFS server has to be able to query the user account to populate the claims information.
A single ADFS server can service all domains in a forest (because all domains in a forest have bi-directional, transitive trusts). In addition, a single ADFS server could service all domains and forests that have a trust relationship. If you have forests without trusts, you need one ADFS server for each forest.
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