The amount of users your account is eligible to depends on your subscription details. To find out how many users you can add to your accounts:
Navigate to Company & Subscription
Select Subscription
On the right hand side, view the status of your subscription.
This highlights how many users you have added, in addition to your max amount.
Additionally, select Billing Details to view your payment method or billing history. Furthermore, invoices can be found under the Invoices tab, also found in Company & Subscription.
If you're looking to increase your maximum amount of users, contact [email protected] and we'll help you get it sorted.