To head off the obvious concerns:

First, as established the first time Matt ran into Stephanie, Matt can see through Stephanie's illusions (sometimes inadvertantly), as Matt has second-hand experience with magic (having lived with Alex as she learned it) and Stephanie isn't really that good at such illusions. However, Stephanie isn't the one making these illusions. The Theatre Orb is doing it. The Orb was designed by mages who are considerably better at this than she is. This, of course, is the entire reason she's trying to steal it in the first place.

Second, Matt clearly KNOWS the numerous Stephanies charging towards him are illusions. After all, he knows what the Orb's for, plus he just saw her using it in front of his face (not to mention there are numerous clones of the same person charging towards him, which is sort of a giveaway). So he's well aware that what he's looking at isn't real.

Except for one of them.

That one is sort of the one that counts. And, he can't tell which one it is.