Complex sentence: It was so popular (more than 2,000 people) that not all those who turned up were able to go to the top of the monument and many were forced to leave their contact details to be given priority on the next open day.
Split into simpler sentences: It was so popular (more than 2000 people), not all those who turned up were able to go to the top of the Monument. Many were forced to leave their contact details and will be given priority on the next open day.