Firstly, congratulations! Twins!
Who you can have with you depends on your anaesthetic - are you having an epidural or spinal?
I had a GA with DS so no one was allowed in (same again this time) and a spinal with DD so my Mum was with me.
I would definitely check on that as usually with an epidural / spinal you can have someone to support you. With a GA, you don't need it, you are asleep (that's what i've told myself!)
With DD there was a lot of people in the room, probably 10 or so, I was a high risk case, not sure if that makes any difference.
If I am honest, the whole experience was lovely, I walked in the operating theatre, met the staff who were all friendly, laid on the bed and they got on with it. They do have to move quickly, so don't let that alarm you, but they will talk you through.
As for pre-op? I didn't have one with my c-sections, but from other operations they usually explain the procedure and sometimes do some blood tests (not sure if this is applicable with a c-section)
Best of luck OP!