Drafted email is stuck

Command + Q will close the Active Application and thus the offending e-Mail too. That or Apple Icon on Desktop >> Force Quit from Menu and Choose Apple Mail

Try this - disconnect computer from the Internet. Open Apple Mail and navigate to offending Draft email and delete. If that works - reconnect to internet.

Should that not work - go www.icloud.com and log into iCloud with iCloud ID and password. Perform the same actions as above.

And . . ?

If you attempt to reproduce the same type of email with 100+ Recipients and get the same result - that should tell you something.

Then, it would be logical to deduce one has exceeded to number of Recipients one can send to on one email.