I thought exactly the same : 8000. Although http://en.wikipedia.org/wiki/Heineken_Music_Hall says 5500.
You can bring earplugs, but they won't be necessary. Sound engineers should stick to the concert rules and keep the sound at a maximum of 100 db, far below ear damage.
A little backpack/bag is usefull. Or a coat around your waist and something to eat and drink in a plastic bag you can throw away after the concert. NOT THE COAT, just throw away the plastic bag. Keep your coat with you. You want to stay in front of the stage, not in front of the cloakroom.
Time to meet? In Westerpark we arrived at 12:00 and it was already crowdy. 11:00?
€6 or €10 for parking?? All carparks in the area use the same prices. I hope a maximum of €10 per day, since we will be there all day . Westerpark was €22 for a day
Please don't forget to practice the flashmob dance