Hello Dan,
As long as the Event is still active you can Edit the Settings section of the Event. This is where you set the limit for any fees or items that are for sale. You should be able to adjust the limit for the particular option and then Save the Event.
Sending out the Email Campaign sends a link to the Landing Page if you have that set up, or the link directs to the Registration Page. Once you Save the Event the Registration Page will update with whatever new limit you have put in place so anyone who visits the page after will see the new limit.
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