Seats and plans
A seat is a person. Your plan sets what one seat includes, and seats pool: the allowance is shared across the team rather than rationed per head.
The allowance pools
Each seat adds its plan's monthly allowance to a single company pool, and everyone draws from that. A developer deep in a migration can use far more than a colleague reviewing pull requests without either of them hitting a personal wall.
That is also why per-person budgets exist. Pooling is the right default for getting work done; a per-member budget is how you put a limit back on one person without rationing everyone.
Adding and removing seats
Seats are changed from your subscription. Adding one is prorated to the day, so you pay for the part of the period you actually use it. Removing one frees it for a future invitation.
Converting to a business account
An individual account can become a business account by adding your company details, which is what puts a company name and address on the invoice rather than a personal one. Converting is free and does not change your plan.
Do this before you invite anyone, not after. It is the difference between an invoice your finance team accepts and one they send back.
The roll-up
A manager view of the team week by week: who moved, what changed, and which seats are barely being used. Idle seats are the cheapest saving available, and this is where they show up.