When you switch to the PRO plan, the price is based on the number of operators that can be online simultaneously. In other words, you pay for so-called operator seats.
Note that you can have any number of registered operators with any plan.
An administrator receiving no chats doesn't take an operator seat. You only pay for those operators who answer customers' questions.
If you need more seats, you can always buy them. To learn more, read the article Add an operator on PRO plan.
For more information, see Plans and pricing FAQ.