Although it differs a lot based on the nature of the email, but for most sales and marketing email, the golden range is between 50 to 125 words. Anything beyond that stands a chance of directly being ignored.
This is not just an intuition but backed by data from Boomerang’s research. There are other studies that claim that anything between 200 words is fine.
If you’re a marketer and wondering how many words does the average email has and optimizing based on that, for a safe bet, keep it below 200 words. However, this is not a hard-rule that needs to be followed. Depending upon the need and requirement of the particular mailer, the number of words can be tweaked.
After all, it’s the effectiveness of the mail that matters and not the number of words.