Not in who can learn it, but certain elements have their own limitations. Fire and ice based spells can only target one enemy except in the really advanced spells while lightning and wind based spells can attack multiple enemies but is limited in range. And sometimes people won't be compatible with certain elements.

I see. That's a little different, since I we mostly just draw energy from within ourselves.

I see. Can more than one person hold a pact with a spirit at the same time?