Was it only one??? *SPOILERS*
I know who is left at the end and Mary Lou looks I tithe camera, smiling, leading us to believe it was her all along.
But the count tells the legend of how it was believed to be a devil that would inhabit the person's body, right? He also states that the person who is the wolf wouldn't change until the full moon and wouldn't be able to control themself. He was wrong about the second part. The wolf had been changing throughout at different times.
What if it was like a devil spirit? Similar to the spirit in part three, remember? And it could only take the form of a person with that mark on their arm? All of them had similar backgrounds where they were orphans and it's hinted or stated that the person who is the wolf may not even know they are the wolf.
Yet that actress knew where the crevice was in the secret passage. When the professor was killed, Mary Lou was around the group of people, not off to herself. The count had that mark ad he disappeared a number of times, not to mention that was HIS button they found by the professor's glasses in the catacombs.
I think the count thought it was only going to be one of them & to find out who, he invited them all to the castle. He had no intention of letting any of them leave though, much like on the opening. He'd kill them all to be sure they got the one. But by bringing them all there he gave the devil wolf spirit a plethora of people into as needed.
<---Bella's "Kiss Me" Face