Step by step
Open Staff
Go to Staff and note how many people are Active versus Inactive.
Check capacity
Look for the Staff capacity / seats indicator on Staff or under Settings → Billing. Your included limit matches the current plan policy (Starter 2, Pro 5, Business unlimited unless the UI shows otherwise).
When you hit the limit
If Add staff is blocked, deactivate someone you no longer need, remove a seat via Billing add-ons when offered, or upgrade. Do not invent seats the Billing page does not show.
Prefer Inactive over Delete
Deactivating frees bookability and can free a seat while keeping the profile for later.