Your subscription plan sets how many staff accounts you can add. Your own owner account does not take a seat.
Check your seats
Open Staff in the side menu. The Total Staff card shows how many staff you have against your limit, and how many seats are left. On a plan with no limit, it says Unlimited on your plan.
When every seat is taken, Add Staff turns grey and you see You have reached your plan's staff limit. Press Upgrade your plan to add more staff to see the plans, or delete a staff member you no longer need to free a seat.
What about drivers?
Drivers do not use staff seats. They have their own Drivers page, and your plan gives them their own driver places. The Driver places card on that page shows how many are left. See Adding a driver and reading their cash ledger.