You definitely can!
1. Click on the Settings button to the left of your event's homepage.
2. You should be directed to the Event Information page. To the right, you can now adjust how many RSVPs you would like to allow. Simply enter how many RSVPs you'd like, then save.
Are you allowing your attendees to bring +1s? If so, you're in luck: +1 guests will be counted towards the total capacity, along with your original invitees. This means that you will have an accurate display of exactly how many guests plan to attend your event.
If the capacity only allows for one more guest, and your invitee tries to register with a +1, they will see the an error letting them know that there is only one more seat left for the event. All error messages can be further customized via: How do I edit the error messages on my form?