Does the GM have to spend a magic token to invoke the magical aptitude of an NPC?  Or to have an NPC invoke the magical abilties of a supernatural item?

This might get out of hand if the had a bunch of vampires for example, each with the aptitude of "Vampire".  Does the GM have to spend a token for each one of them to activate their Vampire ability?  This would seem to seriously cut into the GM's pool of magic tokens.

Yes, the GM needs to spend a magic token to activate an aptitude, too, or to use a supernatural item. For crowds, I usually have the generic "vampire" statted out, and use the same stats for all. In this case, the GM could probably just invoke "vampire" for all of the low-level guys, and separately for the boss.

This isn't specified in the rules, but you have a good point about GM resource management in this case. Of course, the GM can also sacrifice power tokens from his pool for this, just as described in the Power chapter.

Invoking once for mobs of enemies sounds fair.

I understand that the GM can use Power tokens as a substitute for magic tokens that are spent.  Can the GM also use Power tokens to buy new magic tokens?  If the GM hadn't used all of his power tokens by the end of the session, would the rules allow the GM to purchase new magic tokens with them that could be carried over to next session?  The rules allow players to do this, but it isn't clear whether the GM can.  I would imagine that in most sessions the GM would have exhausted most of his Power tokens though.  In a High magic game, it would always be advantageous to buy a new magic token rather than spend a power token to accomplish the same task.

GMs can't buy new magic tokens with power tokens, but they do get additional magic tokens whenever another player buys one.

That's what I thought (about GMs not being able to buy magic tokens), just checking.  Thanks!