The limits are still the same: Can't bind more Foci than your Magic, and can't have more than Magicx5 Force total bound.
What makes sense would be a Power Focus (which boosts both multiple spelltypes and multiple spirittypes, making it worth the cost for a HTR team), one or two Sustaining Foci for buffspells, perhaps a Centering focus to let them cast more powerful spells in combat. Strangely enough there's no Shielding Focus, maybe that's a deliberate nerf because of how Boosted Defense was changed.