By default, Demio does not limit the number of Registrants at all and people are free to sign up to any Session you scheduled. 

However, you can set a limit to the total number of Registrants per Session from the Event settings if needed.

The setting is available from the Customize tab of your Event settings:
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Adjusting the number here will allow you to limit the number of Registrants for each Session in this Event. The registration limit can also only be applied to Standard and Automated Events.

Once the limit is reached, the next Registrant trying to sign up for your Event will see an error message saying that the Session is not available for registration:
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If all upcoming sessions reach their limit, the Registration page would be disabled and use the "event is no longer available" message. This indicates the Event is not available anymore due to all the Sessions being full.