For the sake of argument, we take a wizard with two slot open for Foci, has elements Earth, Wind, Fire. Can that Wizard have a Foci for each element ("This one is +2 to Offensive power for Fire, this is +1 Offensive/Defensive Control for Earth, this one's +2 Defensive Power for Wind."). I hate to sound like a goober, but the rules merely say what your Foci slots are but do not tell whether or not there is a limit to the number of Foci you can have: Only their respective strengths, representing the ability of the Wizard to create such an item. If that's the case, then assuming the Wizard in question has time to do so, s/he can change between Foci ("I'm gonna need the +2 to Defensive Control for this one.") for situations--rather like a golfer selecting his clubs.
And no, I don't want spellslingers bedecked with foci, just wondering if, say, one foci per element absent of a harder ruling than mine be allowed, or if it's even abusive and game-breaking. Maybe a Stunt is called for here (Crafting or Lore, perhaps), to allow for multiple foci?