Hi Alan,
Good question…only thing I could find is this document:
a) It doesn’t specify anything about the number of interfaces, only the number of sessions.
b) For a single session, you specify a source + destination so that’s one session.
c) maximum of 4 (2 if switch is stacked with Catalyst 2960-S switches) source sessions and 64 RSPAN destination sessions. A source session is either a local SPAN session or an RSPAN source session.
Rene