How do I get a second iCloud mail account?

You can create additional email addresses - up to three - as an alias to your existing iCloud email address.

See Apple's support document Add and manage email aliases for iCloud Mail on iCloud.com

Sign into your iCloud account in a web browser, then -

Create an email alias

1. In Mail on iCloud.com, click the action clog at the top of the Mailboxes list, then choose Settings.
2. Click Accounts, then click “Add an alias”.
3. Provide the requested information:

• Alias: The text you provide becomes the email address (alias@icloud.com). An alias must contain between 3 and 20 characters.
• Full Name: The name you provide appears in the From field of the messages you send.
• Label: Aliases are listed alphabetically by label.

4.Click Add, then click Done.

You can add Aliases to your existing one: link-> Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support

Or you ca create entirely new account separate from the one you currently use, from a Mac, iPhone or iPad in Settings. Link-> Settings->Mail->Accounts->Add Account->iCloud->Create new Apple ID.