@Merrilyn5 I'm not sure if you are using Instant Book or not, but if you are, I'd suggest that as a new host, and a single woman renting a private room, that you don't use IB. Make it so guests have to send booking requests. As you have experienced, guests don't bother to read what you have written and is right in front of their eyes. You have made it quite clear in your description that it's a private room in your apartment where you live.
When guests send you a request, communicate with them, making sure they understand that it's a home-share, that you only host 1 guest at a time. Ask them to please read thoroughly through the listing info, including your house rules.
And if anyone shows up with 2, when you've made it clear you only take 1 guest, you have every right to turn them away. You should also be clear about them not inviting other people over to the house- it's a security risk for you. You can always make a decision at the time if they ask, like for instance if a nice guest asks if their sister could come over for a couple of hours to have lunch with them. But don't feel you have to agree to anything that makes you uncomfortable.
The same thing happened to me when I was a new host. My second guest, a young woman, while she realized it wasn't an entire apartment or house, was also a newbie guest, and didn't realize she couldn't have someone else in the room she booked. She sprang her boyfriend on me the first night. I was new, and wasn't sure how to handle it, and they were both very sweet, so while I let her know that you can't book for 1 and then have 2, I let him stay and charged them a bit more for utilities.
When a similar situation happened a few months later, I was more savvy, and told her the male friend couldn't stay.
Another thing about home-sharing. If a guest is doing something that disturbs you, it's important to mention it right away. Don't stew about it and resent it and then end up feeling like you can't wait for them to leave. Or if you wait days to say anything, it will feel like a confrontation you are dreading, instead of saying at the time, "Hey, XX, would you mind not leaving your clothes and stuff strewn around the living room? Gather it up when you go to your room? Let's keep our personal stuff out of the common areas so it doesn't get cluttered. Thanks, I appreciate it."
I've had to let little things not bother me as a home share host- not everyone lives like we do, so you can't be too nitpicky, but if a guest is leaving dirty dishes after their meals or something like that, you have to get that behavior straightened out right away.