If you need to limit the amount of participants in your event, there are multiple options for doing so:
Total Event Registration Limit
This option can be found on the General Event Setup screen, and limits the total number of participants in your event, counting all categories aside from waitlists.
Category Field Limits
When setting up a category, you will see a Field Limit option where you can enter the maximum number of participants in that specific category. This counts online, manual, and reserved entries, and this option will close the category or open the waitlist once that limit is hit.
Category Group Limits
If you have Category Headers in place, you can add a field limit to the category header. This will add all the entries for all categories in that header to hit that limit.