It's not the same crowded as what the park would be on a busy day. I don't know the numbers, but as this is a ticketed event, there are a certain number of tickets they sell and there is a max. I've only been to the Halloween parties, including sold out parties, and I've never had issues with crowds. That being said, I'm also not willing to wait in a line for more than 20 minutes for a character or a ride on party nights, which is definitely different for others. If the hard to see characters are important to you, you'll need to get in line early, but that would be the case regardless of the crowds. Find a spot early for the parade (for us that was only about 20 minutes early and we got right up front). I do recall seeing 7DMT with a long line (45-60 minutes), but my sister and niece rode it and said they only waited about 15 minutes.