What is happening if a room is completed;

Welcome to the Zoom Community, @Athanatos.

Yes, you can register more people than your Meeting Capacity. Very often they are No-Shows – people who register but do not attend. Once you have reached the limit of attendees for your account, additional attendees wi not be able to join.

You can limit the number of Registrations on the Registration tab of your meeting settings. See this Zoom Support article for additional info:

https://support.zoom.us/hc/en-us/articles/211579443-Scheduling-and-customizing-a-meeting-with-registration 

You can also increase the capacity for your meetings with a 1-month Large Meeting add on. See my reply here for additional information on how to obtain this add-on for a 1-month period:

https://community.zoom.com/t5/Billing-Account-Management/how-to-change-meeeting-capacity-for-one-time/m-p/122891/highlight/true#M4787