When you create your MetaMask wallet, you will be given a Secret Recovery Phrase, and an account is automatically generated. You’ll see that as your Account 1 (default account).
If you'd like to create and manage multiple MetaMask accounts, you could do so simply by creating more accounts. Each of these is then associated with the same Secret Recovery Phrase.
If you want to re-add accounts after restoring your wallet using your Secret Recovery Phrase, take a look at this article.
Click the favicon (the circular account image) at the top right of your wallet.
Click 'Create Account'.
Enter your preferred name and then hit 'Create' to confirm.
Either tap the account icon on your wallet home screen, or click on the hamburger icon in the top left and then tap the icon there:
On the account selector menu that appears, tap on 'Create New Account'.
When back on the home screen, long press on the name of the account to change it.
Please note once you add the account there is no way to delete.
To better communicate the importance of never sharing your seed phrase, we've changed the name of the phrase to "Secret Recovery Phrase". Learn more here