I think it's safe to say there's at least two killers, both because that's regular Scream fashion and because it's really the only way to not make the killer obvious by the end of the season, so the killers have alibis for some of the murders. I think there has to be a connection to Brandon James somehow, since I don't see the point in focusing so much on it otherwise. Whether it's that he's somehow still alive and is helping (that would be too Harper's Island for me, but one of the creators was a producer so it's possible) or it's someone related to him.
My current theory is that Brandon James didn't actually commit all those murders back in the day, but it was actually Emma's father, who was stabbed that night and almost died that did it, and he stabbed himself/a partner stabbed him so he could frame Brandon James for it, a nice call back to Billy and Stu's original plan, but this time it was successful. So now the killer is either Emma's father (and someone else), or someone(s) related to Brandon James getting revenge for the murders being pinned on him. I'd have to rewatch the pilot to recall if his body was ever found, I just remember getting shot and falling into the water. I can't imagine all those cops not finding a body in a lake, but Noah did say people were trying to say it was him back from the dead.
I think Audrey will be the leading suspect later in the season and leading up to last episodes, but will end up not being one of the killers. Based on nothing but the fact that I think it would be a nice parallel/reference to the original while not being totally predictable, I want to think it'll end up being both of Emma's love interests, and Will and Kieren are actually working together somehow, for some reason. At least then the dumb triangle would have a purpose and be over after the first season.