I agree. I just think about the boundary conditions when it comes to rules to help clarify things.
If it's okay to have multiple Sprites help on a Test, then that means a TM with Resonance 6 can have 6 Sprites Assist. Therefore, it is feasible (but challenging) for a TM to be able to throw 36 bonus dice (Six Level 6 Sprites would be hard to Register without using Edge) at something and relatively easy to throw 18 *bonus* dice (Six Registered Level 3 Sprites) at something. That would mean probably 30-48 dice, and that is true of a beginning TM.
They are cool, but no one should be that cool.
So, I have to go with one Sprite unless someone can point out a canonical source that explicitly says multiples are okay. And then I need to start devising GM magic to challenge my TM or maybe just catch and vivisect him and have the player roll something else...because if this is correct, then they really belong in the boogeyman category.
With Magicians, the assumption in our campaigns was only one Spirit, one Focus, etc. so I see no reason not to extend it to one Sprite and plenty of reason not to.