I have found a bug in Events Manager 5.2.6.
It is not possible to set "Available Spaces" to sero. If you set "Available Spaces" to 0 (sero) on "Create a Ticket" it will be 10 after saving.
You can solve this by changing the variable declaration in class EM_Ticket in the file classes/em-ticket.php.
Change it to "var $ticket_spaces = 0;"