The Business Central Environments are managed so there is always 1 Production site and up to 3 Sandboxes.

HOWEVER, you can only use the Storage space available. If you have used up all of your available storage for Production and 1 sandbox, you cannot build another Sandbox. You can only use the data that is available to you.

Alternatively, if you have 3 sandboxes already, but still have 40GB of storage left over, you cannot build another Sandbox as you have filled the quota already.

You cannot exceed either limit for data or for number of sites.