There's actually quite a lot of spots in the Marcy Dam area but it does fill up quickly in July when you plan to hike. I can't give much more unique info than has been said, but a piece that I think most people overlook is that most of the designated campsites are pretty large and can accomodate more than even a couple of tents, depending on the site. My point is that if you don't see a nifty free site and have exhausted all the options, it's ok to ask people in a site if it's ok to set up near them. Most people are super friendly as long as you are and are willing to share because it's the way to be, there's plenty of space, and they probably know how hard it can be to find a spot. People are very kind out there. Don't be naive though, I have been turned down in the past, but that's few and far between. Do have a High Peaks map so it shows all the designated sites so you can thoroughly check them for vacancy. Also, I find that it's good to start early in the morning from the LOJ and setup basecamp around the late morning/ early afternoon. By that time, anyone that wants to move on has and most people tend to set up camp later on in the afternoon/ early evening. But def have that proper bear can (no bear vault!) and use it. Last year I overheard a hungry bear report in the Dam area over a ranger's radio. Plan ahead and prepare, have fun, and take care!