The usual clutch size for Eastern Bluebirds is 3-5 eggs, but occasionally you'll see six eggs.
You can find information & tips for monitoring your bluebird nest and see what birds use a bluebird nest box at these links:
Monitoring Bluebird Nestboxes
Common Nesting Birds - https://nestwatch.org/learn/focal-species/
go to our How to Monitor a Nest Box page. The Factsheet linked there shows other birds & their nests that you might see using your nest box.
Contact us any time with questions at bellavistabluebird@gmail.com or call/text Laura at 847-951-1743.