If the person opts to register for more of an item than is available, the system will then not require the item, and will allow the user to finish the registration without selecting/purchasing the item.

For this reason, it is recommended that Registration Items that are required should not also be set to a limited quantity. If overall event registration is limited, it is instead recommended that a single Registration Item be configured to 'Fee Per Attendee Type'.

Fee Per Attendee Type

If this does not fit your circumstances, please see another possibility here:
How did a user go through an Event Registration without paying?