Accounts are your workspaces/teams, and you can have several accounts in your profile. If you are part of more than one account, you will see several icons on the left toolbar on your dashboard. Switch between the accounts by clicking the square icons on the left-side panel:
This user has 10 accounts on their dashboard
Creating a board
By clicking the button New Board you create a board in the account that is chosen at the moment. Thus the features and the number of boards available depends on the account you are in. See the list of features and plans available to compare the capabilities of each plan. Read more about creating a board here.
Board search
You can see only the boards that were shared with you personally or with the whole team. To find a board on the Dashboard, click Search at the top of the page or press Ctrl+F and start typing the board's name:
The search option indicates the account which the board belongs to
Explore the basics of organizing your account with this video: