Life happens and sometimes people will need to reschedule or cancel their appointments. You shouldn’t have a firm policy in place that discourages this; instead, you’ll want to be open to discussing the options and providing people with convenient choices. You can choose to charge a cancelation fee for last-minute cancelations or prevent people from making future appointments if they cancel too much, but make sure that you allow people plenty of time to reschedule or cancel.
Have an emergency policy in place, too, because sometimes there are things out of people’s control and that shouldn’t be held against them.