It's not currently possible to set a registration or attendance limit for specific sessions in HeySummit. Here are some other options:
It may be possible to set such limits in the webinar provider, e.g. Zoom, BigMarker, but keep in mind that this won't apply to HeySummit tickets or registration. A limit set in your webinar provider account will limit attendance, but attendees will still be able to register for sessions in HeySummit.
You can limit the number of passes sold via HeySummit in your ticket settings. Try setting up a ticket that only allows access to a specific session then limit the number available. Check out this doc for more info on ticket settings.