To see an easy view of how many open seats you have in your Organization, when you are in a Workspace click on Settings > Workspace > Billing.
You can see how many users you can add in the Invite New Users, User Directory, or Billing Overview sections.
To find the number of billable users and invites in your Organization, you can also go to the Manage Organization and Workspaces Dashboard (click on your company's logo in the upper right corner, then click on View All Workspaces). Once there, click on Manage Organization and Workspaces.
At the top of the first column in the table, you'll find the total number of billable users and invites in your Organization (the number will be larger and bolded).
You’ll also see a list of all the users in your Organization. Active Billable Users (Admins, Owners and Members) are shown in bold. Non-Billable Users (Disabled or Observers) are shown in italic.