Okay so from ready the base book (not core excett) there is a Magic Level maximum as defined by your intelligence. So an intelligence 10 character would have a maximum Magic Level of 50.
Now it does state that you do not automatically start at your maximum Magic Level and that the book recommends half that amount for Mystic archetypes and a quarter that for those of Mixed Mystic archetypes.
So per my understanding if you have an intelligence of 10 for a maximum of 50 Magic Levels and you were to raise your Path Rating to 30 that would take up 30 of your Magic Levels.
Then if you were to put 20 into the MetaMagic Spheres that would take up the rest of your Magic Levels.
So once this is done you are unable to spend anymore Magic Levels until your intelligence raises so your maximum can raise.
We just recently started up a new campaign and the majority of the party are Mages. So when I went reading through the Magic section this seemed odd but balancing for the magic.