It's been a long time since I did Scarefest, so I could well be wrong. But it seems to change every year. At some point they had a 'core three' set of mazes, plus a fourth one, with the fourth one only being able to bought individually. I think they've moved away from that now, and heavily push the quartet option, but quietly having options for solo mazes - likely at £10 a pop, and possibly even only available to buy on the day?
Long and short of it is that they'll probably advertise if you can buy single maze tickets once they've announced their line up and full offerings.
With it being the 15th anniversary of Scarefest they're looking at pulling out all the stops for it, as it were. But simply though, I'd be surprised if they didn't offer single maze tickets, as it's a great way to squeeze extra money out of people.