When completing a bulk email, there is the possibility of the recipient number being lower than the number of originally selected prospects. This is most likely due to the prospects' contact preferences.
Some prospects may indicate that they do not want to be contacted by email. If they have, a user may have marked them for "Do not email" on the Prospect tab of their profile:
There are toggles for "Do not call", "Do not text", "Do not email", and "Do not mail" on every prospect's profile. If they are toggled on, they will be exempt from bulk emails.
Another reason a prospect may be missing from the recipient list is if a Prospect does not have an email associated with the prospect profile. A prospect will not be included in a bulk email if they do not have email information on their profile.