It's possibile to control the amount of attendees of your event using the Waitlist tool.
If your event has reached the maximum capacity:
everyone who tries to access the registration form, will see the following message:
So, you can define a Waitlist and aprove manually all the new attendees.
First, the tool must be active in the path Event -> Configuration -> Define Tools and serch for "Wait List". Click on the "Edit" button and check the box to activate the tool.
After that, the registration form will be open again and, after the attendee have filled out the form, the following message will appear:
The admin of the event can manage the new admission through the path People -> Registration -> Manage Admissions
Then, just click on the "Approve" button to the applicant you want: