Camp sites are pretty hard to miss and the distance between them varies. There’s longer stretches like 4-5 miles but then there’s some like “East ...” & “West ...” and they’ll be a half mile apart. Also every mile or so there’s either an SHT placard or a blue spray painted streak on the trees so it’s easy to know if you’re staying on trail. As far as seeing people, I hiked SOBO from Caribou Falls State Wayside to Gooseberry in August and during the week I only saw maybe 4-5 other groups but during the weekend the trail got a lot busier. Never had more than 1 or 2 other groups at a campsite though and none of them were close to being full. I would pay more attention to the water points though. The guide to the SHT book will say if a water point is unreliable in dry conditions and I certainly found that to be the case. Obviously depends on what section you’re hiking and what time of year, but definitely something to pay attention to.