You can change the number of spaces available for sale on any of your events at any time, provided they haven't been sold yet.
1) From the Class Profile
Choose the class you wish to edit from My Timetable to view it's profile.
On the left hand side of the profile, there is a list of class dates and the number of spaces currently available in each event.
Click on 'capacity' to make changes.
2) When Viewing the Event Register
If you're already on the register for the event, there is also a status at the top of the register showing the number of 'Places available'. Click on this card to change the number of tickets available.