"Six is fine. You're free to come and go as you like, you know, while you're here. And the books I've given you- read as much or as little as you need to. You'll know what works for you. And when you have some more background information, we can go from there." Merlin gave her a short half-bow with a one-handed flourish, then left the room to go and make sure his potion wasn't simmering dry.
He'd make sure to give Rosario a good fifteen minutes before he came back, to give her the freedom to look around without feeling watched. So long as she didn't go trying to open any cabinets, she should be fine. He would keep an ear open though.