How do I add more emails to my iCloud account on iPhone?

On your iPhone, go to Settings/Apps/Mail/Mail Accounts/Account Mail/Add Account. Choose your email provider (Google, Microsoft Exchange, Outlook.com, Yahoo, etc.), sign in with your email address and password, and follow the instructions. Once the account is added, you can open the Mail app to access messages from all your accounts in one place.

There’s no way to create iCloud.com email aliases directly from an iPhone.

To create additional iCloud Mail addresses (aliases), go to icloud.com/mail and sign in with your Apple Account. Click the Settings button, choose Mail Settings, then Account/Add Alias. Enter the name you want to use for the new @icloud.com address, add a label if desired, and click Add. You can create up to 9 email aliases for your primary iCloud Mail address. Aliases use the same inbox as your primary address and can also be used to send and receive email.

Perhaps you are asking about an email alias as described here What are email aliases in iCloud Mail? - Apple Support ?

This article tells you how you can add up to 9 aliases:

Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support

Create an email alias

1. Go to https://www.icloud.com/mail then sign in to your Apple Account (if necessary).

2. Select the gear icon at the top of the Mailboxes list, then choose Settings.

3. Select Add Alias, then provide the requested information:

• Alias Address: The text you provide becomes the email address ([email protected]). An alias must

contain between 3 and 20 characters.

• Label: Aliases are listed alphabetically by label.

• Full Name: The name you provide appears in the From field of the emails you send.

4. Select Create.

If you are asking about adding and managing the email address for others, this support article may be the one you need. Manage and delete contacts on your iPhone or iPad - Apple Support

Adding addresses may depend upon which email services you are using.