I've done group trips with 20-25 people (mostly teenagers). Some Vermont and Massachusetts state campgrounds have group campsites that are set apart from the rest of the sites (i.e., Shaftsbury State Park in Vermont, and I know there is at least one in the Berkshires). I don't know if NH state campgrounds have group sites; I've only camped in Pittsburgh and I don't think those campgrounds have them. You can get a string of regular sites together, but then you have to be especially careful about noise since families will be camping right next door. Typically, private campgrounds have sites that are not as far apart as public campgrounds, so the noise and privacy issues for other campers can be greater. I usually opt for public campgrounds.
Will you have kids or adults? If kids, I found it very helpful to have a campground with lots of available activities so they aren't sitting around the campsite getting bored and making noise during free time. Last year we took about 25 kids to Branbury State Park in Vermont. We went for a half-day hike, then let them swim at the beach and there was a basketball court as well, and we played soccer in a field on the other side of the campground. That way they didn't make too much noise when they were actually at the campsite, because they were tired out. Organized activities is the key to both crowd control and everyone having a good time. (Keep in mind that many kids, if they aren't required to participate in an organized activity, will choose not to do anything and will be miserable, whereas if they were required to participate they would end up having a good time.)
If using a string of regular sites, I put the picnic tables all together in the central-most site, and we all eat in a group, as far from the edges and other people as possible. We had one central campfire near the picnic tables, rather than multiple campfires at all the sites. Basically this saved on wood and made it easier to chaperone!
Another thing, with a string of regular sites, is to arrange your tents and assign people to their locations. If you know that some of your group will tend to be up later and keep talking after quiet hours, put them in the middle, as far from other campers as possible, and put your quieter people on the edges. Even if you have adults I don't think there is anything wrong with assigning people to locations.
For day hiking activities, have a backup plan for people who can't make the hike. Have some leaders that are willing to lead the turnaround group if someone can't go the whole way. Have some alternative activities back at camp that they can do while the rest of the group is out hiking. Depending on how much your group has ever hiked before, you probably wouldn't want to do more than a 4 mile hike with a group that size; otherwise some of them will almost certainly have a miserable time. I've been continually surprised at how much my groups didn't want to hike, even on what seemed like an easy hike to me. The key is to make sure they have a good time and want to do it again.
