A seat is a person. Your firm buys a number of seats, and every active user occupies one.
Where to see your seat count
Settings → Users & SSO, at the top of the Users tab. It reads like 3 / 11 seats: how many people you have, and how many you are entitled to.
What takes a seat
Every user, whatever their role. A Read-Only user costs exactly the same as a Technical Admin. Limiting someone to a handful of clients does not make them cheaper either.
Two things that catch people out:
An invited person takes a seat immediately, before they have signed in for the first time.
Deactivating someone does not release their seat. The seat stays bought and stays counted. If you are paying per seat, the Users tab tells you so directly, with a line reading something like "You're paying for 2 unused seats."
Releasing a seat is a billing change, separate from deactivating the person.
If your firm is on an agreement
Firms on an enterprise agreement see their plan under Settings → Subscription & Billing: the seats your agreement includes, the term dates, and whether it renews.
Your seat count is fixed by the agreement rather than bought as you go. When you hit it, inviting another person is refused with a message naming your seat count and pointing you at your account manager. Adding seats is a conversation, not a checkout.
Deleting your firm account
The Danger Zone at the bottom of the Subscription page permanently deletes your firm account, cancels the subscription, and anonymizes all user data.
There is no undo. If you want your data first, take a copy before you start. See Export all your firm's data.