1. It is presumably possible for some front end to start up any number
of Maxima processes that may or may not be runnable in parallel on
one or more CPUs or locations on some network.
Whether this is something you want to do or not, it doesn't require
changes in Maxima.
2. In some lisp systems it is possible to harness several CPUs to
do parallel processing in some way. Sometimes in more than
one way. If you want to seriously consider running Maxima using
several processors within the same lisp system, I think you could
pick one of the lisp systems and learn about its process model(s)
and think about use could be made of them.
3. Common Lisp does not specify parallelism.
RJF