You can find a list of all of your portals, associated with the account that you're currently logged into, by clicking on the My portals link in your user menu within one of your portals.
Or by going directly to
[Your portal URL]/admin/account/apps
Your current portal will be highlighted and you are able to navigate to your other portals by clicking on the item.
If you wish to create a new portal, this can be done on the My portals page (see above). Click on the create a new portal button and you will be taken to the portal setup page.
You will be asked to enter your Airtable API key again. Please make sure that you do not re-generate your Airtable API key. Navigate to your Airtable account page and click on the API key input box and it will be revealed for you to copy.