No, there is not an extra fee for more admins on business, pro and enterprise accounts. Pricing is based on the total number of active users -- board creators and contributors -- who use your system (whether they are regular users or you give them admin privileges).
To view the number of active users currently on your plan:
- In the upper right corner click the settings (gear) icon and select Admin Panel.
- The number of active and archived users will show in the Users tab.