Giving a talk to three people is easy. No sweat. Giving it to 100 costs you a night’s sleep.
Sending an email to six colleagues is normal. Sending a note to a list of 400 is cause for concern.
Where, exactly, is the line?
Is an audience of 21 different from 24?
If you spend some time looking for the line, perhaps you’ll discover that there’s rarely a reason to freak out. It’s just one more than the number you’re fine with, after all.