The number of HR users allocated by default is dependent on the plan size selected. See below:

When you set up an account you will automatically be the Admin, and you will also automatically be assigned as one of your HR users.

From there you can assign other members of your team to have HR privileges.

If you are not looking to upgrade your plan we offer the option to purchase an additional 3 HR Users. This can be done by going to Configure > Plan and Billing > Change plan: