Each of your Global Division Mappings can be configured to respect a limit on the amount of registrations in that entire division. Setting a limit on the division will apply for the entire association and all Packages configured in those divisions.
For example, if you set a limit of 25 in Atom, then the maximum amount of registrations in that division will be 25 pre-registrations. Packages configured in the Atom division will be available as long as this global limit has not been reached. If an Atom package itself is configured with a limit of 30, it will never reach that limit since the Global Division limit is set to 25.
To set a limit on the entire division, open the Global Division Mapping you want to configure from the list of Division Mappings on your main Online Registration administration page, enter the limit in the "Registration Limit" box, and click Save.