Pardot automatically deduplicates prospects within your distribution list (if you select multiple email lists, a prospect will only receive one email in spite of possibly being on more than one list). You can also select suppression lists (lists of prospects who SHOULD NOT receive your email). Lastly, Pardot also automatically suppresses anyone who has unsubscribed from emails or has hard bounced (or had five soft bounces). Given all of these suppressions, it can be difficult to predict exactly how many prospects will be sent a list email.
Now you can click a button calculate the total on the distribution screen.
The total distribution size will also be displayed for you automatically on the confirmation screen of the email wizard.