Hi
The simplest way to do this is to have a formula field on reservations which is set to a 1 if status =m Cancelled - you can then use a rollup field on Clients to add up how many times the client has cancelled.
You could make this more sophisticated and work out a percentage of cancellations by using CountAll to count all a clients bookings and work out Cancellations/Bookings*100 to give a percentage.
Hope this helps.
Julian