How do I see how many sessions/classes have been booked, completed or are left, for a particular client?

You can view this information if you head over to the 'Clients' tab, then to the right of your client you should see 'Summary' button underneath 'Info' button. Click this, and it will tell you how many sessions/classes they have booked, how many completed, and how many are left (along with some other handy payment info). Please note that this is reading from your Calendar's bookings, so you would have to book in the sessions/classes first.