To answer this question, it depends whether you are a team member or a guest. To learn about the difference between each type of users, check out this article.
The team members can see all main boards of your account. They can also view the shareable and private boards they have been invited to. Team members can be either members or viewers. To learn more about the difference between each type, check out this article.
Guests can be invited to shareable boards only. You can invite a guest to as many shareable board as you want. To learn more about guests, check out this article.
If you have any questions further questions, please feel free to reach out to our customer success team at firstname.lastname@example.org. We are available 24/7 and happy to assist.