By default, Demio does not limit the number of Registrants at all and people are free to sign up for 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 a message which indicates that the Event is not available anymore due to all the next upcoming Session being full.