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: think of your wallet (your Secret Recovery Phrase) as a container of one or more accounts. (For more detail, see here: What's the difference between a wallet and an account?)
Note that once you add an account, there is no way to permanently delete it. It will exist on the blockchain forever.
If you want to re-add accounts after restoring your wallet using your Secret Recovery Phrase, take a look at this article.
Click the account selector at the top of your wallet.
Click 'Add account'.
Enter your preferred name and then hit 'Create' to confirm.
In wallet view, tap the name of the currently selected account.
On the account selector menu that appears, tap on 'Add account or hardware wallet', and then select 'Add new account'.
The account will be added when you tap. Once created, change its name using the instructions here.
As mentioned above, created accounts are permanent and cannot be removed. They are associated with your Secret Recovery Phrase from the point of creation onwards. If you really want to no longer see them in your wallet, you could restore your wallet (do not do this unless you have your Secret Recovery Phrase recorded somewhere, otherwise you will lose access to all of your accounts). Any accounts that have a zero ETH balance will not be automatically re-added during this process, meaning you'll no longer see them.