This article shows you how to use the Bcc feature in Mail in macOS Sierra (10.12) and later. The option is

Here's how to send a message to Bcc recipients in macOS Mail:

Open a new email window in the Mail app on a Mac. Note: The Bcc field may not appear when you first open a new email window in macOS Mail.

Select View > Bcc Address Field from the menu bar to add the new area.