You can add as many administrators as you need to your instance. The default Admin group created with your instance has full access, so be careful with what users you add to this group. Admins can cancel your account, change payment information, remove events or pages. You get the point.
To make an existing user an admin, log into your account and go to Admin. Then click on Users >> Groups. There you can see all the groups on your instance and create new ones. Find the group named Admin. Inside the admin group on the left you will see the users in the group. Click on the 'Users' title on the left pane. From there a modal will open where you can assign an existing user to the admin group.
To create a new admin, log into your account and go to Admin. Then click on Users >> All Users. There you can see all the users on your instance and create new ones. Click the Create button to create a new user. From there a modal will open where you will enter the user's information. There is also a tab named Groups where you can select the groups to add this new user to. Select the Admin group to add the new user as an administrator -which gives them full access. By default all new users are sent an email to verify and create their password.