The Event Capacity under creating an Event and the Registration Form capacity do not connect to the Participants in any way. This is the root of multiple ideas presented. Those fields should be able to check to make sure that the Participants who are marked with an RSVP Status of Attending were dynamically reducing the Registration Form's(s') availability.
The Event Capacity should be the source of truth for the event. Everything else should be maxed out at whatever that value is set as.
There would also have to be an option on the Registration Form creation to disconnect the Registrants from adding to the Event Capacity. This way you could still have options for Registrations that do not come with physical attendance.
If registrants decide after registering to withdraw, those numbers, currently, are not removed from the Registration Form capacity. This makes staff do manual work to adjust that limit each time something happens like this. If we can just remove their RSVP or change it, this would save up a lot of time since we should be updating those fields, not Registration limits.
See below for knowledgebase articles addressing "event capacity":
https://kb.blackbaud.com/knowledgebase/articles/Article/48651
Based on customer feedback we're providing more granular capacity management for RENXT Events by offering the ability to limit the capacity for each fee.
You can use the "Limit capacity for this fee" option on registration and other fees to limit the number available for each fee. This capacity limit will prevent the fee from being oversold on Event Registration Forms and when manually adding fees to participants in the back-office.
Please check out What's New? and Fees for more information.
Note: The "Event Capacity" field within the "Details" tile on an event is still intended to be used for reference and does not limit the number of participants or registrations submitted through forms.