How many first come first serve camp spots available? Are there any available for the weekend of July 17 & 18th? What is a good number for reserving a spot?

Almost all are good. Usually all pretty close to a bathroom. You do have to reserve at recreation.gov though

All the camp spots in Sand beach campground are no longer first come first served. You must reserve a spot. We just camped there over the 4th of July and had to make reservations for a spot. There is a first-come-first-serve area right before you get out on the dunes.