If they are quoting a price for a private tour, it could very well be the total cost for the group - and that tour would be sold to one set of people signing up together, it would not be "open registration." If they are quoting a per person price, then that is your cost - you will not be expected to add to it if there are less than 10 people who sign up. However, they may cancel (read the small print) the tour if not enough people sign up to make it financially viable.
If they say "up to 10", why would you want to cancel if indeed you could know how many are signed up? Are you saying if there were only 4-5 people on the tour, you wouldn't want to take it?