How can I send same email multiple times in iOS?
A good way to achieve this using just the builtin apps on iOS is to save your email as a Note in the Notes app. You can then Share it by Mail, and it will be formatted for you automatically, with the first line becoming the subject of your email.

You could even setup a Notes folder with a whole set of email templates that way.

Your question pertains to iOS and iPad, but in macOS you may also select a previously sent email, then go into the menu bar and click Message → Send Again or Redirect. These commands are also available through right-click.

Of course the Notes solution still applies in macOS.