I’m not a birth mum, I’m an adoptee and and adoptive parent.
I’ve met my child’s bio mum several times and know lots of other adopters who met at least once . Everyone has said that it was hard but very worthwhile and so helpful for them ( the adopters ). AFAIK most birth family members find it helpful too.
When we met, BM brought a relative with her, her SW was there and also ours. So it felt quite busy IYSWIM. Usually a SW will manage the meeting In some way, it won’t be a free for all.
You don’t have to say much, BM will probably come with some thing to say or questions to ask you.
If yours is a closed adoption, you need to decide in advance what identifying information you are willing to give ( Wasn’t an issue for us as it’s an open adoption ).
BM will often want to make sure that you get “ their side “ of the story that led to the child being adopted. They can understandably be upset or angry when retelling this, but their SW should deal with this. You don’t have to do anything but listen.
They might ask what you are going to tell the child about them / their background.
They might ask about your job , your religion etc. Anything that seems important for them to know about the People who will be bringing up who they see as their child.
Sometimes people ask questions that seem random - do you like sports because I was really good at x sport as a child and I want LO to play it.
I was asked “ you are not Y religion are you because I hate Y “ 
Some BM prefer adopters without bio children because they think that the adoptee will be second best. If you have bio children you might want to assure her that you will treat them all the same, even if you are not asked this directly.
Some will ask you to promise you won’t change the child’s name. If you are going to do so, think carefully about what to say to this.
I know that most AP feel that the birth family are judging them to see if they are fit parents. But in fact most BM are worried that YOU will judge them for failing to care for their child.
So its best if you can try to be good listeners and let BM talk, rather than try to tell her how great parents you will be. Because she knows that, as you have been approved by SS. So you don’t need to rub it in IYSWIM. I’m sure you won’t, but it can come over that way when adopters try to reassure BM that they will raise the child well. It’s well intentioned I know .
You will come away exhausted but reassured I think . Most people are struck by how ordinary and normal everyone else is. BM are mostly not the bad people that social services reports have made then out to be . Adopters are not evil baby snatchers, just people wanting to parent.
Everyone comes to adoption from a place of loss, so you have a lot more in common than you think.
So in summary, just be kind and a good listener.
Once you get home, write down everything because you won’t remember later.