Id card is definitely a must. If you want to visit the prison frequently, you should get a card, but if you are his friend, you can't do it.
There is no limit to the number of people at the reception. You can go as much as you want. Of course, if it is exaggerated to dozens of people, there will still be restrictions for management and safety reasons.
There is a date to visit the prison.
Usually on weekends.