Hi,
They offer them daily at 8:30, 10, 11:30 a.m., 1, 2:30 and 4 p.m. I called about a month in advance and I was the only one booked but then again that was October and it was pretty quiet. The day of, just a few minutes before our ride two others joined me so we were only three! I believe they can take 12 at a time. You could probably call when you get there if you are not going at a really busy time. Leave yourself lots of time to get there... it's easy to get lost. It was fun... I had a great time with Otis (the horse!). Our guide told me he had been the Headless Horsemans Horse last year at some of the MNSVSHP's so I was really thrilled.
Have fun...