There is no limit on the amount of people who register to your event, there is a limit on the number of people who can attend your webinar when its a free session.
Free sessions can have unlimited registrants.
There isn't a limit on the number of people who can REGISTER for your webinar, but there is a limit on the number of people who can ATTEND your webinar if you are hosting a free session.
The number of attendees (or "seats in your session") and how many tickets you can sell for your event is based on the BigMarker plan you choose.
You will not be able to collect registrations past the number of seats you have in your webinar room once you enable payments for your webinar.
More information on how to monetize your events on BigMarker here.